DAILY SLEEP, WEEKLY WORKING HOURS, AND RISK OF WORK-RELATED INJURY: US NATIONAL HEALTH INTERVIEW SURVEY (2004-2008)

 
Chronobiology International, 27(5): 1013–1030, (2010)
                                                                                     Copyright © Informa UK Ltd.
                                                                                     ISSN 0742-0528 print/1525-6073 online
                                                                                     DOI: 10.3109/07420528.2010.489466

                                                                                     DAILY SLEEP, WEEKLY WORKING HOURS, AND RISK
                                                                                     OF WORK-RELATED INJURY: US NATIONAL HEALTH
                                                                                     INTERVIEW SURVEY (2004–2008)

                                                                                     David A. Lombardi,1 Simon Folkard,2,3 Joanna L. Willetts,1 and
                                                                                     Gordon S. Smith4
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                                                                                     1
                                                                                       Center for Injury Epidemiology, Liberty Mutual Research Institute for Safety, Hopkinton,
                                                                                     Massachusetts, USA
                                                                                     2
                                                                                       Laboratoire d’Anthropolgie Appliquée, Université Paris Descartes, Faculté de Médecine,
                                                                                     Paris, France
                                                                                     3
                                                                                       Body Rhythms and Shiftwork Centre, University of Wales Swansea, Swansea, UK
                                                                                     4
                                                                                       National Study Center Trauma and EMS, University of Maryland, Baltimore,
                                                                                     Maryland, USA
                              For personal use only.

                                                                                          The impact on health and safety of the combination of chronic sleep deficits and
                                                                                          extended working hours has received worldwide attention. Using the National
                                                                                          Health Interview Survey (NHIS), an in-person household survey using a multistage,
                                                                                          stratified, clustered sample design representing the US civilian, non-institutionalized
                                                                                          population, the authors estimated the effect of total daily self-reported sleep time and
                                                                                          weekly working hours on the risk of a work-related injury. During the survey period
                                                                                          2004–2008, 177,576 persons (ages 18–74) sampled within households reported that
                                                                                          they worked at a paid job the previous week and reported their total weekly work
                                                                                          hours. A randomly selected adult in each household (n = 75,718) was asked to report
                                                                                          his/her usual (average) total daily sleep hours the prior week; complete responses
                                                                                          were obtained for 74,415 (98.3%) workers. Weighted annualized work-related injury
                                                                                          rates were then estimated across a priori defined categories of both average total daily
                                                                                          sleep hours and weekly working hours. To account for the complex sampling design,
                                                                                          weighted multiple logistic regression was used to independently estimate the risk of a
                                                                                          work-related injury for categories of usual daily sleep duration and weekly working
                                                                                          hours, controlling for important covariates and potential confounders of age, sex,
                                                                                          race/ethnicity, education, type of pay, industry, occupation (proxy for job risk), body
                                                                                          mass index, and the interaction between sleep duration and work hours. Based on
                                                                                          the inclusion criteria, there were an estimated 129,950,376 workers annually at risk
                                                                                          and 3,634,446 work-related medically treated injury episodes (overall injury rate
                                                                                          2.80/100 workers). Unadjusted annualized injury rates/100 workers across weekly
                                                                                          work hours were 2.03 (≤20 h), 3.01 (20–30 h), 2.45 (31–40 h), 3.45 (40–50 h), 3.71

                                                                                         This paper was presented at the 19th International Symposium on Shiftwork and Working
                                                                                     Time, August 2–6, 2009, Venice, Italy.
                                                                                         Address correspondence to David A. Lombardi, PhD, Senior Research Scientist, Center for Injury
                                                                                     Epidemiology, Liberty Mutual Research Institute for Safety, 71 Frankland Road, Hopkinton, MA 01748,
                                                                                     USA. Tel.: (508) 497-0210; Fax: (508) 435-3456; E-mail: david.lombardi@libertymutual.com

                                                                                                                                      1013
1014                             D. A. Lombardi et al.

                                                                                        (50–60 h), and 4.34 (>60 h). With regards to self-reported daily sleep time, the esti-
                                                                                        mated annualized injury rates/100 workers were 7.89 (10 h). After
                                                                                        controlling for weekly work hours, and aforementioned covariates, significant
                                                                                        increases in risk/1 h decrease were observed for several sleep categories. Using 7–7.9
                                                                                        h sleep as reference, the adjusted injury risk (odds ratio [OR] for a worker sleeping a
                                                                                        total of 10
                                                                                        h of usual daily sleep, the OR was marginally significantly elevated, 1.82 (95% CI:
                                                                                        0.96–3.47). These results suggest significant increases in work-related injury risk with
                                                                                        decreasing usual daily self-reported sleep hours and increasing weekly work hours,
                                                                                        independent of industry, occupation, type of pay, sex, age, education, and body mass
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                                                                                        (Author correspondence: david.lombardi@libertymutual.com)

                                                                                        Keywords Epidemiology; Fatigue; Injury; Sleep; Work hours

                                                                                        INTRODUCTION
                                                                                         The impact on health, safety, and well-being of sleep deficits and
                                                                                     extended working hours has received increasing worldwide attention
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                                                                                     (Caruso et al., 2006). With respect to sleep, the evidence is growing that
                                                                                     short sleep duration is associated with several chronic disease outcomes,
                                                                                     such as diabetes (Gottleib et al., 2005), hypertension (Gangwisch et al.,
                                                                                     2006), cardiovascular disease (Ayas et al., 2003), obesity (Di Milia &
                                                                                     Mummery, 2009; Marshall et al., 2008; Singh et al., 2005), and sickness
                                                                                     absences from work (Westerlund et al., 2008). In contrast, long sleep dur-
                                                                                     ations have been associated with adverse mental health conditions, such
                                                                                     as depression (Patel et al., 2006). With respect to safety, lack of sleep has
                                                                                     been associated with traffic crashes (Connor et al., 2001; Teran-Santos
                                                                                     et al., 1999), and work-related injuries (Dembe et al., 2005; Folkard &
                                                                                     Lombardi, 2005). Likewise, long working hours and demanding work
                                                                                     schedules are associated with various adverse impacts on worker safety
                                                                                     and health (Biggi et al., 2008; Dembe et al., 2005; Esquirol et al., 2009;
                                                                                     Hanecke et al., 1998; Lin et al., 2009; Suwazono et al., 2009). One study
                                                                                     of a large representative sample of US workers found that working >60
                                                                                     h/wk increased injury hazard rates by 23% (Dembe et al., 2005), and
                                                                                     another large German study reported an exponential increase in work
                                                                                     “accident” risk observed beyond the 9th h of work (Hanecke et al., 1998).
                                                                                         Injury risks associated with long work hour schedules and overtime
                                                                                     have been shown to vary widely across a range of occupations in the USA
                                                                                     (Dembe et al., 2008). This effect has been demonstrated across multiple
                                                                                     industries; among manufacturing workers, for example, those who worked
                                                                                     >64 h the week before the shift in which they were injured had an 88%
                                                                                     excess risk compared to those who worked
Sleep, Working Hours and Risk of Injury             1015

                                                                                     workers, long work hours and irregular work schedules were significantly
                                                                                     associated with a higher work-related injury rate after controlling for
                                                                                     potential confounders (Dong, 2005). With respect to health outcomes, long
                                                                                     working hours has also been associated with hypertension, cardiovascular
                                                                                     disease, diabetes, and sleep disorders (Härmä, 2003, 2006). However, only a
                                                                                     limited number of published studies that have examined the association
                                                                                     among long work hours, sleep duration, and injury risk have controlled for
                                                                                     important potential confounders, e.g., occupation, industry, and body mass
                                                                                     index (BMI). Many have examined only small study populations making
                                                                                     them vulnerable to the healthy worker effect and limited external validity
                                                                                     (Folkard & Lombardi, 2004). Despite these limitations, there appears to be a
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                                                                                     clear relationship between work hours and sleep duration, with an increase
                                                                                     in work hours being associated with both a reduction in the opportunity to
                                                                                     sleep and sleep quality. In one study involving 5720 Stockholm workers,
                                                                                     high work demands and physical effort at work were found to be risk indi-
                                                                                     cators for disturbed sleep (Åkerstedt et al., 2002), and another study of 367
                                                                                     forestry workers reported that long working hours led to reduced sleep,
                                                                                     compromised recovery time, and potentially reduced work safety (Lilley
                                                                                     et al., 2002). More recently, a study by Virtanen et al. (2009) reported that
                                                                                     working >55 h as compared to 35–40 h/wk significantly increased by 2-fold
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                                                                                     the risk of shortened sleeping hours (odds ratio [OR] = 1.98, 95% confi-
                                                                                     dence interval [CI] = 1.04, 3.77), after adjusting for demographic factors.
                                                                                         With respect to other potential confounders in the relationship
                                                                                     between work hours, sleep duration, and injury risk, a study of the US
                                                                                     population using the National Health Interview Survey (NHIS) data
                                                                                     showed a descriptive association between usual sleep duration and obesity
                                                                                     among adults (Schoenborn & Adams, 2008). The findings of this study
                                                                                     suggested that those sleeping 40 h/wk rose from 26% to 31%.
                                                                                     At the same time, data from the US Centers for Disease Control and
                                                                                     Prevention (CDC) indicates that the percent of males and females reporting
                                                                                     an average of
1016                         D. A. Lombardi et al.

                                                                                     Sweden (Westerlund et al., 2008). We used the National Health Interview
                                                                                     Survey (NHIS) and pooled across a large population of workers from the 5
                                                                                     most recent years of available data (2004–2008) to estimate the independent
                                                                                     effect of usual daily sleep duration and weekly working hours on the risk of
                                                                                     a work-related injury in the USA, controlling for important factors such as
                                                                                     age, sex, race/ethnicity, education, type of pay, industry, occupation (as a
                                                                                     proxy for job risk), body mass index (BMI), and the interaction among
                                                                                     usual sleep duration and work hours.

                                                                                        MATERIALS AND METHODS
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                                                                                        Study Population
                                                                                         The data analyzed in this study are from the NHIS, which is an in-
                                                                                     person household survey with a multistage, stratified, clustered sample
                                                                                     design that provides estimates on health indicators, health care utilization
                                                                                     and access, and health-related behaviors of the civilian, non-institutiona-
                                                                                     lized population residing in the 50 states and the District of Columbia in
                                                                                     the USA. The NHIS has been a continuous nationwide sample survey
                                                                                     since 1957 conducted by the National Center for Health Statistics
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                                                                                     (NCHS) to monitor the health of the population of the USA on a broad
                                                                                     range of health topics (NCHS-NHIS, 2008). The current study was
                                                                                     approved by the Liberty Mutual Research Institute for Safety’s
                                                                                     Institutional Review Board and complies with the ethical requirements of
                                                                                     the journal (Portaluppi et al., 2008).

                                                                                        Pooling
                                                                                         The data for this study were pooled across the 5 most recent years of
                                                                                     the NHIS survey (2004–2008). In general, the survey questions and
                                                                                     sampling design for these years is nearly identical. However, starting in
                                                                                     2006, the size of NHIS was reduced by about 13% due to budget con-
                                                                                     straints (Chen et al., 2009), and a further cut of another 13% was made in
                                                                                     2008 (NCHS-NHIS, 2008). NHIS data are obtained through a complex
                                                                                     sample design that allows for valid population estimates, assuming appro-
                                                                                     priate adjustments are made to the statistical analysis that take into con-
                                                                                     sideration the stratification, clustering, and weighting in the study design
                                                                                     (NCHS-NHIS, 2008). Pooling NHIS data across years for estimation is con-
                                                                                     sidered reliable when the proper adjustments to sampling weights are
                                                                                     made. Sample weights were assigned by the NCHS for each respondent on
                                                                                     the basis of the number and composition of households and included
                                                                                     adjustment for non-response (Chen et al., 2009; NCHS-NHIS, 2008).
                                                                                         Figure 1 describes the selection criteria and sample sizes for the pooled
                                                                                     data used in the current study. In summary, for the years 2004–2008, the
Sleep, Working Hours and Risk of Injury   1017
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                                                                                     FIGURE 1 National Health Interview Survey (2004–2008).

                                                                                     NHIS data includes 417,390 persons with complete sampling design infor-
                                                                                     mation reported in the family survey by an adult in the household. This
                                                                                     family survey includes injury information on 281,451 adults aged 18–74
                                                                                     yrs, of which 177,576 (63%) were reported to be working the previous
                                                                                     week and average weekly working hours was available. Within each
                                                                                     sampled household, one adult was then randomly chosen for a more
                                                                                     detailed interview. Among these adults, 75,718 (93%) provided complete
                                                                                     data on their usual sleep duration, and 74,415 provided complete data on
                                                                                     both sleep and work-hour information. The primary analysis of the
                                                                                     current study includes 69,248 subjects (91.5% of the subsample) who
                                                                                     reported working the previous week, their weekly work hours, usual daily
                                                                                     sleep duration, and complete covariate information.
1018                         D. A. Lombardi et al.

                                                                                        Injury Data
                                                                                         As with the sleep duration and work hours data, injury data are ascer-
                                                                                     tained each year (2004–2008) in the administration of the Family
                                                                                     Questionnaire, which captures any injury or poisoning episode that a
                                                                                     member of the household may have experienced in the 3 months prior
                                                                                     to the interview that required medical attention (Chen et al., 2009; Smith
                                                                                     et. al., 2006; Warner et al., 2000). This includes a phone call or visit to a
                                                                                     doctor or a visit to a hospital, clinic, or the emergency room (outpatient).
                                                                                     Multiple injury reports are captured in that each family member is
                                                                                     allowed to report more than one injury or poisoning episode. Verbatim
                                                                                     text information on each injury includes the body part and how the
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                                                                                     injury happened, which are recorded for further coding at NCHS. Only
                                                                                     injury descriptions which can be coded to the International Classification
                                                                                     of Diseases, Clinical Modification (ICD)-9CM nature of injury codes
                                                                                     800–999 are included in the injury file (includes acute musculoskeletal
                                                                                     conditions but not gradual onset conditions such as tendonitis) (NCHS,
                                                                                     1997; Smith et al., 2006). Although each person is allowed up to 10 sep-
                                                                                     arate injury episodes, for the purposes of the current analysis, a person
                                                                                     was considered injured if they had at least one injury episode reported.
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                                                                                     An injury was further classified as work-related based upon the response
                                                                                     to the question, “What were you doing when the injury happened?” in
                                                                                     which the response was “Working at a paid job.”

                                                                                        Work Hours
                                                                                         An employed person was defined as a subject who responded that
                                                                                     they were “employed in a job or business during the week” before the
                                                                                     interview. This includes those who were engaged in unpaid work in the
                                                                                     family business or farm, excluding personal work in home. Work hours
                                                                                     were estimated as average weekly working hours for each “employed
                                                                                     person” reported by the randomly selected adult respondent of each
                                                                                     household. A priori categories of weekly working hours were defined as:
                                                                                     ≤20, 21–30, 31–40, 41–50, 51–60, and >60 h.

                                                                                        Sleep Duration
                                                                                         Sleep duration was introduced into the NHIS in the Sample Adult
                                                                                     Core in 2004 using the question “On average, how many hours of sleep
                                                                                     do you get in a 24-hour period?” The large sample size in this study
                                                                                     allowed us to examine the effects of sleep in workers across a range of
                                                                                     seven a priori categories defined as: ≤4.99, 5–5.99, 6–6.99, 7–7.99,
                                                                                     8–8.99, 9–9.99, and >10 h.
Sleep, Working Hours and Risk of Injury                1019

                                                                                         Covariates and Potential Confounders
                                                                                         Our analyses controlled for sociodemographic, physical, and job
                                                                                     factors (Dembe et al., 2008; Schoenborn & Adams, 2008; Vgontzas et al.,
                                                                                     2008). Sociodemographic factors included age, sex, education, and race/
                                                                                     ethnicity (Bohle et al., 2008; Di Milia et al., in press; Folkard et al., 2008;
                                                                                     Gander & Signal, 2008). Education was defined as the highest level of
                                                                                     education achieved. Race/ethnicity was categorized as Non-Hispanic
                                                                                     White, Hispanic, Non-Hispanic Black, and Non-Hispanic Other. For
                                                                                     physical factors, BMI was obtained from the Sample Adult Core and
                                                                                     calculated from self-reported height and weight. Job-related factors,
                                                                                     obtained from the Sample Adult Core, included industry, occupation,
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                                                                                     and type of pay (in response to the question “Were you paid hourly?”).
                                                                                     Industry and occupation were coded by the NHIS using the North
                                                                                     American Industrial Classification System (NAICS) and Standard
                                                                                     Occupation Classification (SOC), respectively, based on a verbatim
                                                                                     description of the person’s industry and occupation. We also controlled
                                                                                     for the interaction between a priori work-hour and sleep-hour categories.
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                                                                                         Data Analysis
                                                                                         To analyze the complex survey design data in the NHIS, SAS version
                                                                                     9.1 (SAS Inc., Cary, NC), procedures for survey data were used to incor-
                                                                                     porate the sampling design attributes and weighting into the analyses. For
                                                                                     descriptive analyses, SurveyMeans and SurveyFreq with corrected standard
                                                                                     errors for mean estimation, 95% confidence intervals, weighted frequency,
                                                                                     and percents were calculated. Weighted annualized work-related injury
                                                                                     rates were estimated across a priori defined categories of total daily sleep
                                                                                     hours and weekly working hours also using SAS Proc SurveyFreq.
                                                                                         Multiple logistic regression using the SAS procedure Proc Survey
                                                                                     Logistic was used to estimate the risk of a work-related injury (as a binomial
                                                                                     variable) as a function of daily sleep time, controlling for weekly working
                                                                                     hours, age, sex, race/ethnicity, education, type of pay, industry and occu-
                                                                                     pation, BMI, and interactions. Parameter and odds ratio (OR) estimates
                                                                                     were computed after adjustment for stratification, clustering, and unequal
                                                                                     weighting using the method of maximum likelihood. Taylor series approxi-
                                                                                     mations were used to compute the corrected standard errors and confidence
                                                                                     intervals of the OR (Binder, 1983). An alpha level of .05 was use for statistical
                                                                                     significance for parameter based on the Wald chi-square; however, sociode-
                                                                                     mographic variables were kept in the model for adjustment, regardless of
                                                                                     their statistical significance. To examine the interaction between total daily
                                                                                     sleep hours and weekly working hours, a hierarchically well-formulated
                                                                                     (HWF) logistic regression model (Kleinbaum, 1992) approach, including
                                                                                     both lower order terms of the two-way higher order interaction, was used.
1020                                 D. A. Lombardi et al.

                                                                                           RESULTS
                                                                                           Subject Characteristics
                                                                                          Among subjects interviewed over the 5-yr period who reported both
                                                                                     working the previous week and average weekly work hours (n = 177,576),
                                                                                     54% were male and the mean (±SEM) age of the subjects was 40.6 (±0.03)
                                                                                     yrs (Table 1). The majority of subjects were White, non-Hispanic (69.5%),
                                                                                     followed by Hispanic (14.0%), and Black, non-Hispanic (11.2%).
                                                                                          Table 2 presents the top 10 industries and occupational categories.
                                                                                     Industry employment was primarily distributed among Health Care and
                                                                                     Social Assistance (13.1%), Manufacturing (11.1%), Retail Trade (10.5%),
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                                                                                     TABLE 1 Characteristics of National Health Interview Survey (NHIS): all subjects (2004–2008)

                                                                                                                          Number of subjects         Weighted frequency      Weighted
                                                                                     Characteristic                         interviewed                    (000s)            percent

                                                                                     Total                                      177,576                   649,752              100.0
                                                                                     Sex
                                                                                       Male                                      94,472                   350,902               54.0
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                                                                                       Female                                    83,104                   298,850               46.0
                                                                                     Age (yrs)
                                                                                       18–24                                    22,321                    84,669                13.0
                                                                                       25–34                                    39,944                    145,705               22.4
                                                                                       35–44                                    44,850                    158,742               24.4
                                                                                       45–54                                    42,809                    156,371               24.1
                                                                                       55–64                                    22,952                    86,396                13.3
                                                                                       65 +                                      4,700                    17,869                 2.8
                                                                                     Mean ± SEM                               40.6 ± 0.03
                                                                                     Combined Ethnicity
                                                                                       Non-Hispanic White                       103,623                   451,322               69.5
                                                                                       Hispanic                                 39,571                    91,245                14.0
                                                                                       Non-Hispanic Black                       23,620                    72,946                11.2
                                                                                       Non-Hispanic Other                       10,762                    34,239                 5.3
                                                                                     Education
                                                                                       Did not complete High School              24,874                   71,854                11.2
                                                                                       High School Graduate                      49,687                   180,545               28.2
                                                                                       Some College                              51,151                   192,979               30.2
                                                                                       Bachelor’s Degree                         32,489                   129,438               20.2
                                                                                       Master’s Degree                           11,682                   46,634                 7.3
                                                                                       Professional Degree                       4,463                    17,959                 2.8
                                                                                       Missing                                   3,230
                                                                                     Body Mass Index (BMI)a
                                                                                       Underweight (30)                              18,527                    67,843                24.9
                                                                                       Mean ± SEM                             27.3 ± 0.02
                                                                                       a
                                                                                        BMI was asked only on the Sample Adult Core questionnaire.
Sleep, Working Hours and Risk of Injury                               1021

                                                                                     TABLE 2 Top 10 occupations and industries of study subjectsa

                                                                                                                                          Number of subjects     Weighted     Weighted
                                                                                     Characteristic                                         interviewed      frequency (000s) percent

                                                                                     Total                                                       74,415             278,650         100.0
                                                                                     Industry
                                                                                       Health Care and Social Assistance                          9,641             35,458           13.1
                                                                                       Manufacturing                                              8,037             29,957           11.1
                                                                                       Retail Trade                                               7,441             28,255           10.5
                                                                                       Education Services                                         6,608             25,374            9.4
                                                                                       Construction                                               5,404             19,914            7.4
                                                                                       Accommodation and Food Services                            4,487             16,065            6.0
                                                                                       Professional, Scientific, and Tech Services                4,443             18,177            6.7
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                                                                                       Public Administration                                      3,746             14,214            5.3
                                                                                       Other Services (except Public Administration)              3,662             13,239            4.9
                                                                                       Finance and Insurance                                      3,496             13,733            5.1
                                                                                       Otherb                                                    14,885             55,366           20.5
                                                                                     Occupation
                                                                                      Office and Administrative Support                          10,238             38,294           14.2
                                                                                      Sales and Related                                           7,459             29,004           10.8
                                                                                      Management                                                  6,328             25,286            9.4
                                                                                      Production                                                  5,162             18,180            6.7
                                                                                      Construction and Extraction                                 4,376             15,572            5.8
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                                                                                      Education, Training, and Library                            4,303             16,832            6.2
                                                                                      Transportation and Material Moving                          4,148             14,987            5.6
                                                                                      Healthcare Practitioners and Technical                      3,734             14,606            5.4
                                                                                      Food Preparation and Serving Related                        3,685             13,230            4.9
                                                                                      Building and Grounds Cleaning and Maintenance               3,234             10,341            3.8
                                                                                      Otherc                                                     19,192             73,439           27.2
                                                                                     Paid Hourly
                                                                                       No                                                        30,595             119,341          42.5
                                                                                       Yes                                                       44,349             161,428          57.5
                                                                                       Missing                                                    552
                                                                                        a
                                                                                          Persons aged 18–24 yrs reporting working the previous week, weekly work hours, usual daily and
                                                                                     sleep duration.
                                                                                        b
                                                                                          Other industries include Administrative and Support and Waste Management/Remediation
                                                                                     Services; Transportation and Warehousing; Wholesale Trade; Information; Real Estate and Rental/
                                                                                     Leasing; Arts, Entertainment, and Recreation; Agriculture, Forestry, Fishing, and Hunting; Utilities;
                                                                                     Mining; Armed Forces; Management of Companies/Enterprises.
                                                                                        c
                                                                                         Other occupations include Business and Financial Operations; Installation, Maintenance, and Repair;
                                                                                     Personal Care and Service; Computer and Mathematical; Healthcare Support; Protective Service; Arts,
                                                                                     Design, Entertainment, Sports, and Media; Architecture and Engineering; Community and Social
                                                                                     Services; Legal; Life, Physical, and Social Science; Farming, Fishing, and Forestry; Military Specific.

                                                                                     Education Services (9.4%), and Construction (7.4%); however, most indus-
                                                                                     tries were represented in the NHIS data. The most frequent occupations
                                                                                     were Office and Administrative Support (14.2%), Sales and Related
                                                                                     (10.8%), Management (9.4%), Production (6.7%), and Construction and
                                                                                     Extraction (5.8%).
                                                                                         Most injuries were due to overexertion or strenuous movements
                                                                                     (24.5%), falls (23.3%), being cut or pierced (15.5%), and being struck by
1022                                D. A. Lombardi et al.
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                                                                                     FIGURE 2 Percent of subjects injured by cause (ICD-9-CM E-codes).

                                                                                     object or person (13.0%) (Figure 2). Machinery and transportation
                                                                                     injuries accounted for 3.4% and 2.3% of the injuries, respectively.

                                                                                         Weekly Working Hours and Injuries
                                                                                         Over the 5-yr study period, there were an estimated 3,634,446 work-
                                                                                     related injury episodes, among the approximately 130 million estimated
                                                                                     workers at risk annually (incidence = 2.80/100 workers) (Table 3a).
                                                                                     Unadjusted annualized injury rates generally increased across the seven
                                                                                     categories of weekly work hours: 2.03 (≤20 h/wk), 3.01 (20–30 h), 2.45
                                                                                     (31–40 h), 3.45 (40–50 h), 3.71 (50–60 h), and 4.34 (>60 h).

                                                                                         Usual Sleep Duration and Injuries
                                                                                         The unadjusted estimated annualized injury rates/100 workers gener-
                                                                                     ally increased with decreased duration of usual daily sleep (Table 3b): 7.89
                                                                                     (10 h). Thus, the crude (or unadjusted)
Sleep, Working Hours and Risk of Injury                           1023

                                                                                     TABLE 3a Estimated annual injury incidence/100 workers by weekly work hours

                                                                                                    Average         Number of        Est. number of
                                                                                     Hours          number            workers         work-related      Est. number        Est. annual
                                                                                     worked/        of hours        interviewed          injuries        of workers         incidence/
                                                                                     week           worked           2004–08a           annuallyb         annually         100 workers

                                                                                     Total            40.3           177,576           3,634,446        129,950,376           2.80
                                                                                     ≤20 h            14.9            14,785             229,343         11,286,527           2.03
                                                                                     21–30 h          27.2            13,333             298,900          9,929,180           3.01
                                                                                     31–40 h          39.3           101,442           1,746,467         71,388,048           2.45
                                                                                     41–50 h          47.4            28,396             761,163         22,042,456           3.45
                                                                                     51–60 h          57.9            13,448             387,346         10,433,206           3.71
                                                                                     >60 h            72.1             6,172             211,227          4,870,959           4.34
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                                                                                       a
                                                                                         Includes individuals reporting employment at a job or business and who had reported weekly
                                                                                     work hours.
                                                                                       b
                                                                                        A work-related injury was reported by 1,178 (0.70%) workers interviewed.

                                                                                     TABLE 3b Estimated annual injury incidence/100 workers by usual daily sleep duration

                                                                                                       Average         Number of       Est. number of
                                                                                                      number of       workers with      work-related     Est. number       Est. annual
                                                                                     Usual hours      hours slept     sleep hours          injuries      of workers at      incidence/
                              For personal use only.

                                                                                     slept/day          (± SE)        2004–2008a          annuallyb      risk annually     100 workers

                                                                                     Total           7.00 ± 0.004        75,271          1,669,142        56,349,239          2.96
                                                                                     ≤4.99 h         3.80 ± 0.010         1,431             83,730         1,061,759          7.89
                                                                                     5–5.99 h        5.00                 4,407            172,462         3,310,950          5.21
                                                                                     6–6.99 h        6.00                17,251            469,756        12,988,074          3.62
                                                                                     7–7.99 h        7.00                25,950            448,458        19,774,245          2.27
                                                                                     8–8.99 h        8.00                22,604            413,942        16,571,317          2.50
                                                                                     9–9.99 h        9.00                 2,361             39,101         1,759,342          2.22
                                                                                     >10 h          10.50 ± 0.030         1,267             41,694           883,551          4.72
                                                                                       a
                                                                                         Includes individuals reporting employment at a job or business and who had been asked and
                                                                                     reported usual sleep hours.
                                                                                       b
                                                                                        A work-related injury was reported by 544 (0.74%) workers interviewed.

                                                                                     rate-ratio comparing those sleeping a total of 10 h of usual daily sleep
                                                                                     had a marginally elevated OR. The adjusted injury risk OR for a worker
1024                                  D. A. Lombardi et al.

                                                                                     TABLE 4 Multivariate logistic regression model, adjusted∗ odds ratio estimates

                                                                                                                                                   Standard                   Est. odds
                                                                                     Parameter                                       Estimate        error        p value       ratio

                                                                                     Intercept                                       −5.3859        0.4737
Sleep, Working Hours and Risk of Injury                               1025
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                                                                                     FIGURE 3 Multivariate logistic regression adjusted odds ratio estimates for usual sleep duration.
                              For personal use only.

                                                                                     FIGURE 4 Multivariate logistic regression adjusted odds ratio estimates for weekly work hours.

                                                                                     further examination (Figure 4) suggests that there is generally a 2-fold
                                                                                     risk across work hour categories in comparison to working ≤20 h/wk,
                                                                                     except for the 30–40 h/wk category, where the OR = 1.54 or a 54%
                                                                                     increase in work-related injury risk is observed. These can also be seen in
1026                        D. A. Lombardi et al.

                                                                                     Table 3a, in examining the unadjusted annualized injury rates/100
                                                                                     workers across categories of weekly work hours: 2.03 (≤20 h/wk),
                                                                                     3.01 (20–30 h), 2.45 (31–40 h), 3.45 (40–50 h), 3.71 (50–60 h), and 4.34
                                                                                     (>60 h).

                                                                                        DISCUSSION
                                                                                          This study of the adult population of the USA pooled data across
                                                                                     the 5 most recent years of the NHIS survey (2004–2008) and found that
                                                                                     both sleep duration and weekly working hours are significantly and
                                                                                     independently (i.e., there was no significant interaction between these
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                                                                                     factors) associated with the risk of a work-related injury. Decreases in
                                                                                     usual sleep duration and increases in weekly working hours both
                                                                                     increased injury risk, even after controlling for several important socio-
                                                                                     demographic, job-related, and physical factors. Thus, reduced sleep
                                                                                     increased injury risk, regardless of the number of hours worked/week
                                                                                     and, conversely, hours/week worked increased injury risk, regardless of
                                                                                     usual daily sleep duration. If the potential impact of these risk factors is
                                                                                     considered simultaneously, the independent additive risk of a work-
                              For personal use only.

                                                                                     related injury could be substantial. For example, comparing a person
                                                                                     working >60 h/wk and a usual daily sleep duration of
Sleep, Working Hours and Risk of Injury             1027

                                                                                     response speed typically being reduced and the frequency of errors and
                                                                                     omissions increased when individuals are fatigued (Williamson et al.,
                                                                                     in press).

                                                                                        Strengths and Limitations
                                                                                          An important strength of the current study is its strong external val-
                                                                                     idity due to the use of pooled NHIS data, which are representative of the
                                                                                     adult population of the USA. Pooling of 5 yrs of NHIS data also provided
                                                                                     a large sample size, providing strong statistical power to both evaluate our
                                                                                     primary hypotheses and to effectively control for several important
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                                                                                     potential confounders. The use of NHIS data, and the ability to identify
                                                                                     each injury for work-relatedness, overcomes many of the shortcomings
                                                                                     inherent in other large national data sources (Smith et al., 2006). Our
                                                                                     study used a 3-month recall period for self-reported medically treated
                                                                                     injuries, which may underreport some injuries (Warner et al., 2005).
                                                                                     However, although injury recall improves with increasing severity, it is
                                                                                     unlikely to result in differential underestimation of injuries (Harel et al.,
                                                                                     1994; Smith et al., 2006). Self-reported sleep duration has been reported
                              For personal use only.

                                                                                     in some studies to systematically overestimate more objectively measured
                                                                                     sleep time (Lauderdale et al., 2008; Lockley et al., 1999), and daily varia-
                                                                                     bility in sleep time makes it difficult to integrate over long periods. In
                                                                                     addition, usual sleep patterns may not be representative of the sleep dur-
                                                                                     ation or sleep quality at the time of a reported injury; however, we have
                                                                                     no information in the current study on the timing of the sleep episode(s),
                                                                                     which might be important with respect to injury risk. With respect to
                                                                                     weekly work hours, a weekly average may not reflect the highest risk
                                                                                     periods for an injury, e.g., time on task, and in the current cross-sectional
                                                                                     study, we have no estimate of exposed and nonexposed person’s time to
                                                                                     other potential work hazards. Another potentially important limitation of
                                                                                     this study is that the NHIS does not include information on the particu-
                                                                                     lar work shift of each individual, e.g., permanent night versus rotating
                                                                                     shiftworkers; thus, we cannot evaluate whether shiftwork mediates the
                                                                                     relationship between sleep duration and injury risk. However, the
                                                                                     Sample Adult Core of the NHIS collects individual-level data on industry,
                                                                                     occupation, and type of pay, which were controlled for in the logistic
                                                                                     regression model. Although imperfect, these work-related variables may
                                                                                     provide limited control for shiftwork as a variable, as they are likely
                                                                                     correlated with typical worker exposure to shiftwork (e.g., construction
                                                                                     workers), however not for the type of work shift. Also, BMI was calculated
                                                                                     from self-reported height and weight, there may be some inaccuracy in
                                                                                     our measure of body mass. However, it is unlikely that height and weight
                                                                                     would be differentially reported by injured and uninjured persons
                                                                                     (Breslow & Smothers, 2005).
1028                                  D. A. Lombardi et al.

                                                                                          Conclusions
                                                                                         Workplace health and safety can be improved by reducing worker
                                                                                     fatigue and risk. In the current study both sleep duration and weekly
                                                                                     working hours were independently associated with the risk of a work-
                                                                                     related injury. However, when considering the impact of various work
                                                                                     schedules on injury risk, it is important to take into account the com-
                                                                                     ponents of weekly work hours, such as the length of the individual shifts,
                                                                                     the type of shift, breaks, and number of successive shifts worked (Folkard
                                                                                     & Lombardi, 2005), and importantly, the opportunity for sufficient sleep
                                                                                     duration needed for the adequate rest and recovery of the worker.
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                                                                                          ACKNOWLEDGMENTS
                                                                                         This work was supported by the Liberty Mutual Research Institute
                                                                                     for Safety. The authors are grateful to Dr. Mary F. Lesch and Dr. Yueng-
                                                                                     hsiang (Emily) Huang for their thoughtful comments on this manuscript.

                                                                                        Declaration of Interest: The authors report no conflicts of interest.
                              For personal use only.

                                                                                     The authors alone are responsible for the content and writing of the
                                                                                     paper.

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