Shift Work Exposure Research Articles

Mismatch of Sleep and Work Timing and Risk of Type 2 Diabetes.

OBJECTIVETo examine whether a mismatch between chronotype (i.e., preferred sleep timing) and work schedule is associated with type 2 diabetes risk.RESEARCH DESIGN AND METHODSIn the Nurses’ Health Study 2, we followed 64,615 women from 2005 to 2011. Newly developed type 2 diabetes was the outcome measure (n = 1,452). A question on diurnal preference ascertained chronotype in 2009; rotating night shift work exposure was assessed regularly since 1989.RESULTSCompared with intermediate chronotypes, early chronotypes had a slightly decreased diabetes risk after multivariable adjustment (odds ratio 0.87 [95% CI 0.77–0.98]), whereas no significant association was observed for late chronotypes (1.04 [0.89–1.21]). Among early chronotypes, risk of type 2 diabetes was modestly reduced when working daytime schedules (0.81 [0.63–1.04]) and remained similarly reduced in women working <10 years of rotating night shifts (0.84 [0.72–0.98]). After ≥10 years of shift work exposure, early chronotypes had a nonsignificant elevated diabetes risk (1.15 [0.81–1.63], Ptrend = 0.014). By contrast, among late chronotypes, the significantly increased diabetes risk observed among day workers (1.51 [1.13–2.02]) appeared largely attenuated if their work schedules included night shifts (<10 years: 0.93 [0.76–1.13]; ≥10 years: 0.87 [0.56–1.34]; Ptrend = 0.14). The interaction between chronotype and shift work exposure was significant (Pinteraction = 0.0004). Analyses restricting to incident cases revealed similar patterns.CONCLUSIONSIn early chronotypes, type 2 diabetes risk increased with increasing duration of shift work exposure, whereas late types had the highest diabetes risk working daytime schedules. These data add to the growing body of evidence that workers could benefit from shift schedules minimizing interference with chronotype-dependent sleep timing.

The ageing shift worker: a prospective cohort study on need for recovery, disability, and retirement intentions.

This study investigates whether different shift work schedules, compared to day work, are associated with need for recovery (NFR), future disability, and retirement intentions for employees employed within different economic sectors over the course of their careers. Shift work exposure duration and the healthy worker effect are also examined. Data from the prospective Maastricht Cohort Study was used. Subsamples of industry (N=1877, all men) and healthcare (N=818, 624 women and 194 men) workers were separately investigated. GEE and Cox regression analyses were performed to investigate NFR longitudinally. Future disability was investigated using Cox regression, and retirement intentions were investigated using logistic regression analysis. Three-shift industry workers were at risk of becoming a case of elevated NFR during follow-up, compared to industry day workers. Three- and five-shift industry workers were at risk for future disability. In healthcare, irregular shift work was a risk factor for disability among older shift workers. No significant results were found regarding retirement intentions. Findings were probably an underestimation as exposure duration to shift work and the healthy worker effect affected the results. Shift work was associated with higher levels of NFR and a higher risk of disability. However, shift work is a multifaceted concept as different types of shift work schedules are differently associated with these outcomes. Different shift work types exist and shift work schedules allow for optimization, indicating that measures to prevent adverse outcomes should be tailored for different types of shift work and over the course of the work career.

Shift rotation, overtime, age, and anxiety as predictors of offshore sleep patterns.

Shift work on offshore oil/gas installations necessitates 12 h shifts and rapid day/night shift changes. In the North Sea, both 'fixed-shift' (alternate day-shift and night-shift tours) and 'swing-shift' rotations (with a midtour shift change) are operated. The present study used survey data (n = 775) to examine sleep patterns over 3 'phases' of the offshore work cycle (day shifts, DS; night shifts, NS; and leave weeks, LS) in relation to shift roster, overtime, age, offshore shift work exposure, and anxiety. Specific predictions were tested in a mixed-model ANOVA in which DS, NS, and LS sleep were treated as repeated measures. Sleep duration and sleep quality were predicted by significant interactions of phase with roster, anxiety, age, and shift work exposure, but the patterns of findings differed across DS, NS and LS. Consistent with other published findings, personnel working 2-week nights-to-days swing shifts reported shorter DS and NS sleep duration than those working fixed shifts. Extended 3-week tours (7 nights/14 days) showed an advantage only for DS sleep. There was no evidence that LS sleep was impaired following night-shift work. Overtime was negatively related only to NS sleep duration. Anxiety predicted poor NS and DS sleep; the relationship between age and NS sleep quality was curvilinear with minimum values at 38-42 y. Shift work exposure negatively predicted NS (but not DS or LS) sleep. The results are discussed in relation to the initial predictions; more general implications of the findings, and methodological limitations of the work, are considered in a final section.

Lifetime shift work exposure: association with anthropometry, body composition, blood pressure, glucose and heart rate variability

ObjectiveTo evaluate the association between lifetime exposure to shift work and blood pressure, fasting glucose (FG), anthropometric variables, body composition and heart rate variability (HRV).MethodsMale shift workers (N=438) were evaluated...

Assessment of exposure to shiftwork mechanisms in the general population: the development of a new job-exposure matrix

ObjectiveTo develop a job-exposure matrix (JEM) that estimates exposure to eight variables representing different aspects of shiftwork among female workers.MethodsOccupational history and shiftwork exposure data were obtained from a population-based...

0219 The association between shift work and obesity in Canada: A cross-sectional study using a novel exposure assessment tool

ObjectivesShift work entailing circadian rhythm disruption is linked to chronic disease. One suggestion is that obesity mediates the relationship, yet research investigating the link between shift work and obesity report...

0422 Coronary artery disease mortality among workers exposed to carbon disulfide and shift work at a chemical manufacturing plant

ObjectivesPrevious studies at a New York State chemical manufacturing plant reported elevated risks of cardiovascular disease among workers. We updated the mortality experience of 1874 workers employed between 1949 and...

0359 Nighshift work and breast cancer risk – good news, bad news?

ObjectivesUsing two large prospective data sets, we explored associations between rotating nightshift work and breast cancer risk.MethodAmong 193 396 women from the Nurses’ Health Study (NHS and NHS2) cohorts, we...

Coronary artery disease and cancer mortality in a cohort of workers exposed to vinyl chloride, carbon disulfide, rotating shift work, and o-toluidine at a chemical manufacturing plant.

We updated through 2007 the mortality experience of 1,874 workers employed at a New York State chemical manufacturing plant between 1946 and 2006. Reassessed exposures to vinyl chloride, carbon disulfide, and shift work and categories of o-toluidine exposure were based on year, department and job title. Standardized mortality ratios (SMR) compared mortality to that of the US population. Internal comparisons used directly standardized rate ratios. Hepatobiliary cancer mortality was elevated among workers ever exposed to vinyl chloride (SMR = 3.80, 95% confidence interval 1.89-6.80); directly standardized rates increased with increasing vinyl chloride exposure duration. No increase in non-Hodgkin lymphoma mortality was observed with vinyl chloride and shift work exposures. Internal comparisons showed increased coronary artery disease mortality among long-term workers exposed to carbon disulfide and shift work for 4 years or more. Excess coronary artery disease mortality confirms earlier results; further investigation is needed to understand risk factors.

Melatonin and stable circadian rhythms optimize maternal, placental and fetal physiology

Research within the last decade has shown melatonin to have previously-unsuspected beneficial actions on the peripheral reproductive organs. Likewise, numerous investigations have documented that stable circadian rhythms are also helpful in maintaining reproductive health. The relationship of melatonin and circadian rhythmicity to maternal and fetal health is summarized in this review. Databases were searched for the related published English literature up to 15 May 2013. The search terms used in various combinations included melatonin, circadian rhythms, biological clock, suprachiasmatic nucleus, ovary, pregnancy, uterus, placenta, fetus, pre-eclampsia, intrauterine growth restriction, ischemia-reperfusion, chronodisruption, antioxidants, oxidative stress and free radicals. The results of the studies uncovered are summarized herein. Both melatonin and circadian rhythms impact reproduction, especially during pregnancy. Melatonin is a multifaceted molecule with direct free radical scavenging and indirect antioxidant activities. Melatonin is produced in both the ovary and in the placenta where it protects against molecular mutilation and cellular dysfunction arising from oxidative/nitrosative stress. The placenta, in particular, is often a site of excessive free radical generation due to less than optimal adhesion to the uterine wall, which leads to either persistent hypoxia or intermittent hypoxia and reoxygenation, processes that cause massive free radical generation and organ dysfunction. This may contribute to pre-eclampsia and other disorders which often complicate pregnancy. Melatonin has ameliorated free radical damage to the placenta and to the fetus in experiments using non-human mammals. Likewise, the maintenance of a regular maternal light/dark and sleep/wake cycle is important to stabilize circadian rhythms generated by the maternal central circadian pacemaker, the suprachiasmatic nuclei. Optimal circadian rhythmicity in the mother is important since her circadian clock, either directly or indirectly via the melatonin rhythm, programs the developing master oscillator of the fetus. Experimental studies have shown that disturbed maternal circadian rhythms, referred to as chronodisruption, and perturbed melatonin cycles have negative consequences for the maturing fetal oscillators, which may lead to psychological and behavioral problems in the newborn. To optimize regular circadian rhythms and prevent disturbances of the melatonin cycle during pregnancy, shift work and bright light exposure at night should be avoided, especially during the last trimester of pregnancy. Finally, melatonin synergizes with oxytocin to promote delivery of the fetus. Since blood melatonin levels are normally highest during the dark period, the propensity of childbirth to occur at night may relate to the high levels of melatonin at this time which work in concert with oxytocin to enhance the strength of uterine contractions. A number of conclusions naturally evolve from the data summarized in this review: (i) melatonin, of both pineal and placental origin, has essential functions in fetal maturation and placenta/uterine homeostasis; (ii) circadian clock genes, which are components of all cells including those in the peripheral reproductive organs, have important roles in reproductive and organismal (fetal and maternal) physiology; (iii) due to the potent antioxidant actions of melatonin, coupled with its virtual absence of toxicity, this indoleamine may have utility in the treatment of pre-eclampsia, intrauterine growth restriction, placental and fetal ischemia/reperfusion, etc. (iv) the propensity for parturition to occur at night may relate to the synergism between the nocturnal increase in melatonin and oxytocin.

Response to Letter to the Editor, re: Ijaz S, et al. “Night-shift work and breast cancer – a systematic review and meta-analysis”

We thank Dr Stevens et al (1) for their comments and interest in our work. We also thank the primary study authors who wrote this letter for their contribution and assistance during the course of our review in 2012. We would not have been able to achieve the quality in our analysis without their input. We certainly agree with their opinion on the current state of evidence being less than ideal and needing more rigor. Thus we believe that their arguments support our conclusion that, based on the low quality of exposure data and the difference in effect by study design, our findings indicate insufficient evidence for a causal relationship between night-shift work and breast cancer. We used recommended guidelines for systematic reviews employing state-of-the-art, valid methods (2–4). One of the key features is that studies of sufficient clinical and statistical homogeneity are combined (5). Clearly the studies that did not fit our predefined criteria for night-shift work based on Stevens et al (6) were not included in the meta-analysis and conclusions. Variation in exposure definition and assessment was further taken into account in risk of bias assessment and did not change results in a subsequent sensitivity analysis. Further bias in our review was avoided by ensuring all decisions on the handling of data were made in advance (a priori protocol) thus separating tested prior hypotheses from ones generated by the data (post hoc). We formulated clear hypotheses about the association between exposure to shift work and breast cancer, which we tested as far as possible given the limitations of the data in the primary studies. We agree that misclassification of exposure is possibly at work in these studies but this would not always be non-differential. Even when this is the case, it is not easy to predict the final result of this bias when the exposure is not dichotomous, as is the case with shift work (7). In the absence of real evidence, we are not convinced that recall bias can be ruled out completely from case–control studies except when the exposure was measured objectively in advance of disease occurrence. Evidence exists that systematic bias occurs in peoples’ reporting of their shift-work experience (8). Most areas of medicine today follow the hierarchy of evidence, preferring prospectively followed-up data to assert or refute causal relationships (9–11), and it should be the same here. We do not know if a long-term risk exists or not because, until Grundy et al’s (12) recently published findings, studies have not used an evidence-based ­a-priori classification of exposure time but rather a data-driven approach to analysis. Following the Grundy et al study, apparently long-term has been defined as >30 years of exposure; we recommend this be further explored. As Stevens et al (1) point out, other meta-analyses came to different conclusions. This is the result of using different inclusion criteria, different methods for combining studies, and the risk of bias assessment. Per GRADE’s (13) recommendation, we used a systematic approach in grading the quality of the evidence. As a result, we rated the evidence as being of very low quality, which means we have little confidence in the results. The authors are also correct in saying that more primary studies find a positive than a negative link. However this method of evidence synthesis, also called vote counting, is widely known to lead to incorrect conclusions. Repeating more meta-analyses and primary studies is not going to bring us closer to the truth, unless the issues in previous research are redressed by improved methods in the future, guidelines for most of which exist in the literature. To this end, a research group recently proposed the idea of conducting an individual participant data (IPD) meta-analysis using the available primary studies. We welcome this initiative as it can put to rest some doubts and save valuable resources. An alternative could be to pool/connect all databases that contain information on working hours and analyze these together as a large single study for hypotheses defined a-priori. We believe that both shift-work exposure and breast cancer are important public health issues that deserve better studies to underpin decisions on causes and solutions.

Exposure to Shift Work as a Risk Factor for Diabetes

Using telephone survey data from 1111 retired older adults (≥65 years; 634 male, 477 female), we tested the hypothesis that exposure to shift work might result in increased self-reported diabetes. Five shift work exposure bins were considered: 0 years, 1-7 years, 8-14 years, 15-20 years, and >20 years. Shift work exposed groups showed an increased proportion of self-reported diabetes (χ(2) = 22.32, p < 0.001), with odds ratios (ORs) of about 2 when compared to the 0-year group. The effect remained significant after adjusting for gender and body mass index (BMI) (OR ≥ 1.4; χ(2) = 10.78, p < 0.05). There was a significant shift work exposure effect on BMI (χ(2) = 80.70, p < 0.001) but no significant gender effect (χ(2) = 0.37, p > 0.50).

388 Exposure assessment for a Canadian census cohort study of night shift work and cancer risks

ObjectivesShift work has been categorised as a probable human carcinogen by the International Agency for Research on Cancer. This is an exposure assessment for a study of shift work and...

Aberrant DNA methylation of miR‐219 promoter in long‐term night shiftworkers

The idea that shiftwork may be carcinogenic in humans has gained widespread attention since the pioneering work linking shiftwork to breast cancer over two decades ago. However, the biomolecular consequences of long-term shiftwork exposure have not been fully explored. In this study, we performed a genome-wide CpG island methylation assay of microRNA (miRNA) promoters in long-term night shiftworkers and day workers. This analysis indicated that 50 CpG loci corresponding to 31 miRNAs were differentially methylated in night shiftworkers compared to day workers, including the circadian-relevant miR-219, the expression of which has been implicated in several cancers. A genome-wide expression microarray assay was carried out in a miR-219-overexpressed MCF-7 breast cancer cell line, which identified 319 differentially expressed transcripts. The identified transcriptional targets were analyzed for network and functional interrelatedness using the Ingenuity Pathway Analysis (IPA) software. Overexpression of miR-219 in MCF-7 breast cancer cells resulted in accentuated expression of apoptosis- and proliferation-related anti-viral immunodulators of the Jak-STAT and NF-κβ pathways. These findings suggest that long-term night shiftwork exposure may lead to the methylation-dependent downregulation of miR-219, which may in turn lead to the downregulation of immunomediated antitumor activity and increased breast cancer risk.

Polysomnographic Sleep and Circadian Temperature Rhythms as a Function of Prior Shift Work Exposure in Retired Seniors.

In an earlier published telephone interview study (n > 1,000) we have shown that retired shift workers subjectively report worse sleep than retired day workers. This laboratory study sought to determine whether these findings held up when objective polysomnograhic (PSG) measures of sleep were taken and whether retirees' circadian temperature rhythms differed as a function of shift work exposure. All completers of the telephone interview were invited to attend a 36-hour laboratory study for which participants were paid. This involved continuous core body temperature measurement (using an ingestible pill-based system) and 2 nights of PSG. Shift work exposure (plus other measures) was collected by taking a detailed work history. The second laboratory night was scored into sleep stages. Post hoc, we divided participants into 4 shift work exposure groups: 0 years (ie, no exposure to shift work), 1 to 7 years, 7 to 20 years, and >20 years. Sample sizes were 11, 16, 15, and 15, respectively, with approximate equality in mean age (71.7 years of age, 69.1 years of age, 70.0 years of age, and 70.4 years of age, respectively) and percent male (63%, 50%, 67%, and 73%, respectively). Shift work exposure was associated with worse PSG sleep in a dose-related fashion. The percentages of participants with sleep efficiency, 80% for the 0 years, 1 to 7 years, 7 to 20 years, and >20 years groups were 36%, 63%, 67%, and 73%, respectively (P < 0.01), and the percentages with total sleep time (TST), 6 hours were 36%, 56%, 53%, and 73%, respectively (P < 0.01). From the circadian rhythm record, shift work exposure appeared to result (P = 0.06) in an increased spread of phase angles (difference between habitual bedtime and time of temperature trough). In conclusion, it appears likely that shift work may be related to a scarring of sleep and circadian rhythms. This may be associated with a change in the relationship between habitual sleep timing and the phase of the circadian pacemaker.

Persistent Rotating Shift Work Exposure Is a Tough Second Hit Contributing to Abnormal Liver Function Among On-Site Workers Having Sonographic Fatty Liver

To investigate the relationship between elevated serum alanine-transaminase (e-ALT) and persistent rotating shift work (p-RSW) among employees with sonographic fatty liver (SFL), the authors performed a retrospective analysis on a cohort of electronics manufacturing workers. The records of 758 workers (507 men, 251 women) with initially normal ALT and a mean age of 32.9 years were analyzed. A total of 109 workers (14.4%) developed e-ALT after 5 years. Compared with those having neither initial SFL nor p-RSW exposure, multivariate analysis indicated that employees who had initial SFL but without p-RSW finally had a higher risk (odds ratio = 2.9; 95% confidence interval [CI] = 1.7-5.1) for developing e-ALT; workers with baseline SFL plus p-RSW had a 3.7-fold increased risk (95% CI = 1.8-7.5). SFL poses a conspicuous risk for the development of e-ALT, and persistent p-RSW exposure significantly aggravates the development of e-ALT among on-site workers with preexisting SFL.

Shiftworkers report worse sleep than day workers, even in retirement

The aim of this study was to explore how the level of shiftwork exposure during an individual's working life might be related to subjectively reported sleep quality and timing during retirement. Telephone interviews regarding past employment and sleep timing and quality (among other variables) were conducted using a pseudo-random age-targeted sampling process. Subjective sleep quality was assessed using a telephone version of the Pittsburgh Sleep Quality Index. Timing of reported habitual bedtimes and rise-times were assessed using the Sleep Timing Questionnaire. Questions measuring morningness and subjective health were also given. Retired seniors (aged >65years, n=1113) were studied. Analysis was by analysis of variance, with shiftwork exposure in three bins [0 (n=387), 1-15 (n=371) and >15years (n=355)], gender (n=634 male, 479 female) and former occupation [in two broad categories, 'managerial' (n=437) versus 'other' (n=676)] as factors. In retired shiftworkers, relative to retired day workers, past exposure to shiftwork was associated with higher (worse) PSQI scores by 1.0 units (1-15years) and 0.6 units (>15years) (main effect P=0.005). There were also main effects of gender and former occupation (males and managerials reporting better sleep), but neither variable interacted with shiftwork exposure. The timing of current mean habitual bedtimes and rise-times (and also the variance around them) were very similar for the three shiftwork exposure groups. The shiftwork exposure effect did not appear to be mediated by either morningness or current health. Prior exposure to shiftwork would appear to be related to currently reported sleep problems during retirement.

Highlights of This Issue

Dogs serve as attractive systems for identifying the genes associated with certain cancers, especially cancers that are difficult to study in humans but that occur frequently in certain dog breeds. For example, histiocytic sarcoma is rare in humans but occurs in 15% to 25% of Bernese Mountain Dogs. To examine the genetics of histiocytic sarcoma, Shearin and colleagues analyzed genomic DNA from Bernese Mountain Dogs, and report a cancer-associated locus spanning the MTAP and CDKN2A genes. The locus is within a region homologous to human chromosome 9p21, which has been implicated in several human cancers. This study demonstrates the power of examining distinctive malignancies in predisposed dog breeds.Although increased breast cancer risk has been observed in night shift workers, the reasons for this are unclear. Peplonska and colleagues investigated the hypothesis that rotating night shift work is associated with disruptions in melatonin synthesis and increased mammographic density. They measured the urinary 6-sulfatoxymelatonin (MT6) levels and mammographic density in 640 night shift workers and found no significant associations between rotating night shift work exposure and mammographic density or between MT6 levels and mammographic density. The results of this study suggest that the development of breast cancer in rotating night shift workers is not mediated by increased mammographic density.Autopsy studies reveal a reservoir of small, occult, undiagnosed breast cancers in women who have died from unrelated causes. To better understand the behavior of these occult tumors, Santen and colleagues used iterative and mathematical techniques to develop a model of occult tumor growth (OTG). The OTG model identified a 200-day effective doubling time, a 7% prevalence rate, and a 1.16-cm detection threshold. The model was validated through comparison of predicted incidence rates with those observed in population databases. The results of this study suggest that occult breast tumors are prevalent, grow slowly, and are the biologic targets of antiestrogen therapy.Lung cancer is the leading cause of cancer mortality. Chest X-rays continue to be used as lung cancer screening tests even though low-dose computed tomography (LDCT) has been found to be more sensitive in early lung cancer detection. Doria-Rose and colleagues estimated the prevalence of chest X-ray and LDCT use in the United States based on data from the 2010 National Health Interview Survey. They report that although LDCT screening rates are low, approximately 8.7 million adults in the United States would be eligible for LDCT screening. Patient education may help ensure that lung cancer screening is used appropriately in those most likely to benefit.

Rotating Night Shift Work and Mammographic Density

An increased risk of breast cancer has been observed in night shift workers. Exposure to artificial light at night and disruption of the endogenous circadian rhythm with suppression of the melatonin synthesis have been suggested mechanisms. We investigated the hypothesis that rotating night shift work is associated with mammographic density. We conducted a cross-sectional study on the association between rotating night shift work characteristics, 6-sulfatoxymelatonin (MT6s) creatinine adjusted in a spot morning urine sample, and a computer-assisted measure of mammographic density in 640 nurses and midwives ages 40 to 60 years. The associations were evaluated using regression models adjusted for age, body mass index, menopausal status, age at menopause, age at menarche, smoking, and the calendar season of the year when mammography was conducted. The adjusted means of percentage of mammographic density and absolute density were slightly higher among women working rotating night shifts but not statistically significant [percentage of mammographic density = 23.6%, 95% confidence interval (CI), 21.9%-25.4% vs. 22.5%, 95% CI, 20.8%-24.3%; absolute density = 23.9 cm(2), 95% CI, 21.4-26.4 cm(2) vs. 21.8 cm(2), 95% CI, 19.4-24.3 cm(2) in rotating night shift and day shift nurses, respectively). There were no significant associations between the current or cumulative rotating night shift work exposure metrics and mammographic density. No association was observed between morning MT6s and mammographic density. The hypothesis on the link between rotating night shift work, melatonin synthesis disruption, and mammographic density is not supported by the results of the present study. It is unlikely that the development of breast cancer in nurses working rotating night shifts is mediated by an increase in mammographic density.

Periodic self-rostering in shift work: correspondence between objective work hours, work hour preferences (personal fit), and work schedule satisfaction

The main objective of the present study was to investigate relative personal fit as the association between rated needs and preferences for work hours, on the one hand, and actual work hours, on the other hand, in three groups (hospital, call-center, and police) working with periodic self-rostering. We also examined the association between personal fit and satisfaction with the work schedule and preference for a fixed and regular shift schedule, respectively. We collected questionnaire data and objective work hour data over 6-12 months from the computerized self-rostering system. The response rate of the questionnaire was 69% at the hospital and call-center and 98% among the police. In total, 29 433 shifts for 285 shift workers were included in the study. Data was analyzed by means of mixed ANOVA, Kendal tau correlations and ordinal (proportional odds) logistic regression. The results show that evening types worked relatively more hours during the evening and night hours compared to morning types as an indication of relative personal fit. Relative personal fit was also found for long shift, short rest, and morning-, evening- and night-shift frequency, but only personal fit related to morning, evening and night-shift was associated with satisfaction with work hours. Reported conflicts at the workplace about work hours and problems with lack of predictability of time for family/leisure activities, was associated with poor satisfaction and a preference for a fixed shift schedule. The present study shows that periodic self-rostering is associated with relative personal fit, in particular with respect to night, evening, and morning work. Personal fit seems to be associated with satisfaction with work hours and may be a moderator of tolerance to shift work exposure.