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© 2009 American Public Health Association
DOI: 10.2105/AJPH.2008.158501
RESEARCH AND PRACTICE |
Orfeu M. Buxton is with the Department of Medicine, Brigham and Women's Hospital, Boston, MA, and the Division of Sleep Medicine, Department of Medicine, Harvard Medical School, Boston. Lisa M. Quintiliani and Glorian Sorensen are with the Center for Community-Based Research, Dana-Farber Cancer Institute, Boston, and the Department of Society, Human Development and Health, Harvard School of Public Health, Boston. May H. Yang and Anne M. Stoddard are with the New England Research Institutes, Watertown, MA. Cara B. Ebbeling is with the Department of Pediatrics, Harvard Medical School, and the Department of Medicine, Children's Hospital, Boston. Lesley K. Pereira is with the Center for Community-Based Research, Dana-Farber Cancer Institute, Boston.
Correspondence: Correspondence should be sent to Orfeu M. Buxton, PhD, Division of Sleep Medicine, Harvard Medical School, Brigham and Women's Hospital, 221 Longwood Ave, BLI-438, Boston, MA 02115 (e-mail: orfeu{at}hms.harvard.edu). Reprints can be ordered at http://www.ajph.org by clicking on the "Reprints/Eprints" link.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONHuman Participant ProtectionReferences |
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Objectives. We assessed whether adequate sleep is linked to more healthful eating behaviors among motor freight workers and whether it mediates the effects of workplace experiences.
Methods. Data were derived from a baseline survey and assessment of permanent employees at 8 trucking terminals. Bivariate and multivariate regression models were used to examine relationships between work environment, sleep adequacy, and dietary choices.
Results. The sample (n = 542) was 83% White, with a mean age of 49 years and a mean body mass index of 30 kg/m2. Most of the participants were satisfied with their job (87.5%) and reported adequate sleep (51%); 30% reported job strain. In our first model, lack of job strain and greater supervisor support were significantly associated with adequate sleep. In our second model, educational level, age, and adequate sleep were significantly associated with at least 2 of the 3 healthful eating choices assessed (P < .05). However, work experiences were not significant predictors of healthful food choices when adequate sleep was included.
Conclusions. Adequate sleep is associated with more healthful food choices and may mediate the effects of workplace experiences. Thus, workplace health programs should be responsive to workers' sleep patterns.
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INTRODUCTION |
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Epidemiological evidence has consistently demonstrated that inadequate sleep duration and compromised sleep quality, independent of other known contributing factors, confer additional risks for weight gain1–4 and obesity5–8 as well as chronic disease. Such findings are of concern given the apparent decrease in the average sleep duration of US adults from 8.5 hours in the 1960s to only 7 hours per night on average by 2000.9–11 Sleep adequacy can be broadly defined as a combination of sufficient sleep duration and sleep quality. Laboratory studies of sleep physiology suggest that, among most adults, adequate sleep duration is approximately 7 to 9 hours.12–14 Large-sample research among adults has shown that those who sleep approximately 7 hours per night are at the lowest risk of mortality.10
Recent research from both laboratory-based and epidemiological studies indicates that sleep restriction is associated with increased hunger and appetite.9,15 One study in which individuals were randomized to 2 nights of sleep restriction or to adequate sleep showed that, after control for food intake, subjective appetite and hunger were elevated across the entire waking day among those with inadequate sleep, and this elevation was strongly associated with increased ghrelin (a peripheral hunger signal from the stomach to the hypothalamus) and reduced circulating leptin (a peripheral satiety signal from the adipocytes to the hypothalamus), which together drive appetite.16
A recent controlled clinical study of sleep inadequacy among middle-aged individuals imposed sleep durations of 5.5 hours or 8.5 hours per night, with food freely available to participants. This study demonstrated that the leptin and ghrelin hunger drive was normalized, or the same, in both conditions because the participants with 5.5 hours of sleep time ate on average about 200 kcal (837 kJ) more of food, primarily in the form of snacks, further supporting a role for adequate sleep duration in the control of eating behavior.17 Complementary cross-sectional epidemiological evidence links short sleep duration with the physiological signal of decreased satiety and increased hunger (reduced leptin and increased ghrelin levels), which is associated in turn with increased body mass index.7
Additional physiological changes in metabolism among young men resulting from sleep restriction include insulin resistance and hormonal changes18–21 that increase the likelihood of obesity, especially visceral adiposity. Although epidemiological and field studies have linked sleep difficulties with job strain and negative workplace experiences,15,22 workplace effects on dietary preferences are less well understood.
Motor freight workers often work long hours, and a variety of factors influence their sleep duration and quality: irregular shifts, mealtimes, and sleep patterns; unsatisfactory sleeping accommodations; and anxiety over traffic, schedules, and economic pressures.23–26 Typically they are on duty for 10 or more hours broken by 8 hours of rest that may or may not provide adequate time and circumstances for sleep. Many drivers work close to the maximum number of hours permitted under the rules of the US Department of Transportation because compensation schedules are tied to work completed. Drivers frequently drive at night to avoid traffic delays and deliver cargo on time. According to one study, a greater percentage of professional drivers (40%) than workers in other job categories (18%) reported experiencing high job strain.27
The "obligatory vigilance" of extended driving is also a stressful component of the occupation and may cause fatigue.28 Work practices for professional drivers may influence obesity rates and chronic disease risk, and a larger proportion of male truck drivers than the general adult male population are overweight.29 A US Department of Transportation survey showed that 90% of truck drivers are at least overweight and 50% are obese, which is approximately double the prevalence of obesity among men in the general adult population.30
We examined the important role of work experiences in both sleep adequacy and dietary choices among motor freight workers given that, as noted, these individuals are at elevated risk of inadequate sleep and poor dietary patterns. We used the social contextual model of health behavior as a framework to describe the roles that work experiences and job conditions play in shaping workers' health behaviors, including dietary patterns.31,32 Although sleep is a health behavior, obtaining adequate sleep can be affected by socioeconomic factors and workplace experiences, among many other factors. In this study, we tested the hypothesis that adequate sleep is linked to more healthful eating behaviors among motor freight workers and mediates the effects of workplace experiences on diet (Figure 1). We predicted that work experiences, including supervisor support, job satisfaction, and job strain, would be associated with sleep adequacy and that sleep adequacy would be associated with dietary patterns.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONHuman Participant ProtectionReferences |
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The aim of the Gear Up for Health Study, which involved a quasi-experimental, pretest–posttest design, was to test an intervention promoting tobacco use cessation and weight management among unionized truck drivers and dockworkers. The data described here were derived from the baseline survey administered at 8 participating trucking terminals.
Sample
Eight participating trucking terminals were randomly selected from 17 eligible terminals in 4 states in the eastern United States (Pennsylvania, Maryland, North Carolina, and New Jersey). Eligible terminals were affiliated with the Motor Freight Carriers Association, an industry trade association, and employed 75 to 150 workers each who were members of the International Brotherhood of Teamsters. Eligible workers were employed for at least 15 hours per week, were permanent employees and International Brotherhood of Teamsters members, and had not been out of work on workers' compensation for more than 2 weeks at the time of the survey.
Participants were employed as over-the-road truck drivers, pickup and delivery truck drivers, or dockworkers or were employed as both truck drivers and dockworkers. Over-the-road truck drivers drive heavy trucks for extended periods of time and cover long distances, pickup and delivery truck drivers deliver freight within a local area, and dockworkers load and unload freight from trucks.
The self-administered baseline survey was conducted on-site in each of the participating terminals between November 2005 and August 2006. Of 697 eligible workers, 542 completed the baseline survey (a response rate of 78%). A standard protocol was used to measure participants' height and weight at the time of survey completion.
Measures
Dietary outcome measures.
We examined 3 food choice categories: fruits and vegetables, drinks with added sugar, and sugary snacks. In addition to being selected as a result of their potential to influence weight, these foods had been identified in preliminary qualitative research conducted for the overall intervention as relevant to the dietary patterns of motor freight workers. Participants reported fruit and vegetable intake by consumption in 6 categories: 100% orange or grapefruit juice, other 100% juices, other fruit, green salad, potatoes (excluding fried potatoes), and vegetables (excluding salads).33,34
Participants also indicated (through the use of a single item) how often they consumed the following: drinks with added sugars (such as regular soda, sport drinks, coffee, iced tea, lemonade, and fruit punch) and sugary snacks (such as cake, sweet rolls, pastry, donuts, cookies, brownies, pie, and candy). In the case of each food, there were 10 response categories ranging from never to 5 or more times a day.
Work factors. Job strain was assessed with an abbreviated version of the Job Content Questionnaire (the 3 subscales used to compute job strain).35 We used 5 items to assess psychological job demand (conflicting job demands, job requires working hard, asked to do excessive amount of work, don't have enough time, and job requires working fast), 3 items to assess decision authority (a lot of decisions on my own, little freedom to decide, and a lot of say about what happens on job), and 5 items to assess skill discretion (job requires learning new things, job involves repetitive work, job allows for creativity, job requires a high level of skill, and job involves a variety of different things).
A decision latitude variable was created as a weighted sum of decision authority and skill discretion. Workers were defined as experiencing job strain if their level of psychological demand was greater than the national median and their decision latitude was below the national median. National medians36,37 were rescaled to adjust for the different number of items used in our study.
We used workers' self-reported schedules to assess job shift. Participants were categorized as working the day shift or working another shift (e.g., night shift, irregular shift, or rotating shift). Mandatory extra work hours, the obligation to work additional hours when required, was measured as a dichotomous variable (whether or not working extra hours were required by the employer). Both of these items were derived from the National Institute for Occupational Safety and Health's 2002 General Social Survey Quality of Worklife Questionnaire.38
We assessed supervisor support by summing responses to the following items: "my supervisor is concerned about the welfare of those under him or her," "my supervisor pays attention to what I am saying," "my supervisor is helpful in getting a job done," "my supervisor is successful in getting people to work together," and "I am exposed to hostility from my supervisor" (reverse scored). Response categories ranged from strongly disagree to strongly agree and were recoded such that a higher score indicated greater supervisor support. Possible scores ranged from 5 to 20 (Cronbach 
= 0.86).
Job satisfaction was assessed with the item "How satisfied are you with your job?" Response options ranged from very satisfied to very dissatisfied. Participants were grouped into 2 response categories: those who reported that they were very or generally satisfied and those who reported that they were somewhat or very dissatisfied. Single-item measures have been shown to have acceptable correlations with more detailed scales measuring job satisfaction.39
We used the item "How concerned are you about being exposed to hazards on your job?" to assess concerns about hazards on the job. Responses were grouped into 3 categories: not at all or a little concerned, moderately concerned, and very concerned.40 We assessed meals brought to work from home by inquiring about the numbers of primary meals workers brought to work from home per week.
Sleep. Sleep adequacy was assessed with the following item: "How often during the past 4 weeks did you get enough sleep to feel rested upon waking up?" Response options ranged from never to very often. We dichotomized sleep adequacy in our analyses by combining the often and very often response categories and the never, rarely, and sometimes categories.
Participant characteristics. The demographic variables assessed included educational level, race/ethnicity, and age. Education was coded as some college or more versus high school or less, and race/ethnicity was coded as White versus non-White. Height and weight (measured at the time of the initial interview) were used to calculate body mass index.
Statistical Analyses
To evaluate the conceptual framework (Figure 1), we used step-down analyses to develop multivariate models. Linear modeling methods were used in conducting all analyses, with trucking terminal as a random effect and all other participant characteristics as fixed effects. We initially explored factors associated with sleep adequacy without including food choice measures. We then modeled the 3 food choice measures and included sleep adequacy as a predictor. For sleep adequacy, we used mixed-effects logistic regression analyses in which clustering of workers in terminals was controlled through inclusion of terminal as a random effect. Initially we evaluated the associations of each worker characteristic alone with sleep adequacy, controlling for terminal only. We then developed a multivariate model including terminal and all measures that were statistically significant individually (P 
.05). Next, we removed variables that were no longer significant and tested for significant interaction effects.
In the case of food consumption, we used general linear mixed-effect models. Again, we initially evaluated each worker characteristic in relation to each food, controlling only for the clustering of workers in trucking terminals. In the multivariate models, we included variables that were significantly associated with any of the 3 food choices in all 3 models. Variables that were not marginally associated with outcomes in the initial multivariate models were subsequently dropped to create more parsimonious models. We then added sleep adequacy to the final food consumption models. Intraclass correlations between the diets of workers within the same terminal and the diets of those in other terminals ranged from 0.01 to 0.02. The intraclass correlation for sleep adequacy was 0.02.
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RESULTS |
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Sample characteristics are displayed in Table 1. All of the respondents were men; the mean age of the sample was 48.6 years (SD = 8.4). Eighty-three percent of the respondents were White, and 28% had completed at least some college. The majority of the respondents (88%) expressed satisfaction with their job; 30% reported job strain. Sixty-six percent of the participants were pickup and delivery drivers, 20% were over-the-road drivers, and 15% were dockworkers. Overall, 52% of the respondents reported adequate sleep.
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Work Experiences, Job Characteristics, and Sleep
Table 1 also shows the association of sleep adequacy with participant characteristics and work factors. In bivariate analyses, White race/ethnicity, job strain, and working extra hours were all associated with lower odds of adequate sleep, whereas greater supervisor support, lower job strain, and being satisfied with one's job were associated with greater odds of adequate sleep. In the multivariate analyses, the association with job strain and supervisor support remained, but race/ethnicity and working extra hours were no longer significant.
Work Experiences, Sleep Adequacy, and Diet
Table 2 shows associations of the food measures with each worker characteristic alone after control for the clustering of workers in terminals. In bivariate analyses, sleep adequacy and race/ethnicity were the only 2 measures significantly associated with all 3 food choices. Non-White respondents and those reporting adequate sleep consumed more servings of fruits and vegetables and fewer servings of sugary drinks and snacks than White respondents and those not reporting adequate sleep, respectively. On average, White participants consumed 0.7 fewer servings of fruits and vegetables per day, 0.31 more servings of sugary drinks per day, and 0.2 more servings of sugary snacks per day than non-White participants. Those reporting often or very often getting enough sleep consumed 0.7 more servings of fruit and vegetables per day, 0.43 fewer servings of sugary drinks per day, and 0.27 fewer servings of sugary snacks per day than those reporting adequate sleep less often.
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Other factors associated with higher fruit and vegetable consumption included working fewer hours and bringing more meals to work from home. Higher educational level, older age, lack of job strain, and being satisfied in one's job were associated with lower consumption of sugary drinks. Similarly, higher educational level, older age, and lack of job strain were significantly associated with lower consumption of sugary snacks, as was higher levels of supervisor support.
In the first multivariate food model, which excluded sleep adequacy (Table 3), non-White race/ethnicity and bringing more meals to work from home were associated with increased fruit and vegetable consumption. Higher educational level and older age were significantly associated with decreased consumption of sugary drinks and snacks; job strain was associated with increased consumption of sugary drinks. When sleep adequacy was added to the model (Table 3), the same patterns of fruit and vegetable consumption and consumption of sugary snacks were observed. In addition, adequate sleep was associated with increased fruit and vegetable consumption. Reports of adequate sleep, along with higher educational level and older age, were significantly associated with decreased consumption of sugary drinks. The slope coefficient for job strain decreased slightly and was no longer significantly associated with consumption of sugary drinks after the addition of sleep adequacy to the model.
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In the final multivariate model (Table 3), job strain, number of hours worked, supervisor support, and job satisfaction were not significantly associated with any of the food choices and were removed to create more parsimonious models. Non-White race/ethnicity, bringing more meals to work from home, and adequate sleep were associated with increased consumption of fruits and vegetables. Higher educational level, older age, and adequate sleep were associated with reduced consumption of sugary drinks and snacks. Only sleep adequacy was significantly associated with all of the dietary consumption variables. The slope coefficients changed very little when the work characteristic variables were removed from the models, indicating only minimal confounding between sleep adequacy and work characteristics in their association with the dietary consumption measures.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONHuman Participant ProtectionReferences |
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In our initial models, we observed that positive workplace experiences such as low job strain and perceived supervisor support were associated with more healthful food choices. However, addition of sleep adequacy in a multivariate model eliminated the statistical significance of workplace factors, supporting the hypothesis that adequate sleep among motor freight workers is linked to more healthful eating behaviors and mediates the effects of workplace experiences on dietary choices.
Sleep adequacy was intended to quantify the subjective experience of sleep, encompassing both subjective sleep duration and sleep quality. Sleep duration and sleep quality are contributing factors to increasing chronic disease trends, in that insufficient sleep duration and sleep disruption have been linked to weight gain,4,41 diabetes,42,43 hypertension, cardiovascular disease,2,44 and early mortality45,46 in the long term. Our cross-sectional findings suggest that sleep adequacy, by enhancing healthful dietary choices, is one means by which workplace factors may influence chronic disease risk.
Limitations
Limitations of this study include the cross-sectional nature of the analysis, which precluded us from making statements about the direction of effects, and the subjective nature of the variables with the exception of body mass index. Our self-reported dietary questions were brief to be feasible in the context of this field study and may not provide valid estimates of actual intake; however, they may be appropriate for measuring associations with other variables, as was done in this study.34 Future work could assess these dietary variables with a more extensive questionnaire or dietary recall system. Although our sleep adequacy question has been used before, a clinical or objective assessment of adequate sleep duration would be preferred for future studies.
Sleep disorders, especially sleep-disordered breathing, were not quantified in this sample. The prevalence of sleep-disordered breathing among motor freight workers has been estimated from high to extremely high in various samples,47 and it is associated with excessive daytime sleepiness even with apparently sufficient sleep durations because the frequency of arousals from sleep across the night leads to the sleep obtained being less restorative. Aside from nicotine, which was included in the model with the smoking variable, consumption of wake-promoting agents (e.g., caffeine and other stimulants) was not assessed in this sample; these agents, which are common factors potentially reducing overall sleep duration, are purposefully taken when sleep is perceived as inadequate. Regular consumption of stimulants would tend to reduce appetite but would not necessarily modify healthful choices in terms of fruits and vegetables or sugary snacks and drinks.
Implications
Epidemiological evidence has consistently demonstrated that inadequate sleep duration and compromised sleep quality, independent of other known contributing factors, confer additional risks for weight gain1–4 and obesity,5–8 diabetes,42,43,48–51 hypertension52 and cardiovascular disease,45,53,54 and early mortality.45,53–55 Our results suggest that employees' diets are associated with their workplace experiences. Diet can directly affect employee health and, therefore, employer risk relative to the health care, absenteeism, and other costs associated with ill health. Future interventions designed to improve employee health via changes in workplace practices might benefit from including positive employee health behaviors, such as obtaining adequate sleep and making healthy dietary choices, as outcome measures given their tangible benefits with respect to employee health and employer interests.
Comprehensive workplace programs designed to promote worker health would benefit from promoting work experiences that include reports on whether workers obtain adequate sleep. The work experiences of motor freight workers provide an illustrative case example of the challenges that work schedules and shifts may pose to adequate sleep. Policies and practices supportive of adequate sleep may have implications for promoting more healthful dietary patterns, as shown here.
Conclusions
In our sample of motor freight workers, positive workplace experiences (lower job strain levels and higher levels of supervisor support) were associated with more healthful dietary choices. However, inclusion of self-reported sleep adequacy in our multivariate model eliminated the statistical significance of workplace experiences. In this sample, adequate sleep was associated with more healthful food choices and may have mediated, in part, the effects of positive workplace experiences on dietary patterns.
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Human Participant Protection |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONHuman Participant ProtectionReferences |
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All of the study procedures were approved by the Dana-Farber Cancer Center's institutional review board. Participants provided informed consent before completing the survey, when they were informed that survey completion was voluntary and confidential.
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Footnotes |
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Peer Reviewed
O. M. Buxton led the writing of the article. L. M. Quintiliani and C. B. Ebbeling, along with the other authors, assisted with data interpretation and the writing of the article. M. H. Yang and A. M. Stoddard conducted the statistical analyses. L. K. Pereira assisted with the collection of data. G. Sorensen originated and supervised the study.
Accepted for publication May 28, 2009.
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONHuman Participant ProtectionReferences |
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