Smoking among shift workers: More than a confounding factor

In studies on the cardiovascular disease risk among shift workers, smoking is considered to be a confounding factor. In a study of 239 shift and 157 daytime workers, it was found that shift work was prospectively related to increased cigarette consumption, indicating that smoking might be in the causative pathway; however, the number of study subjects was too low to warrant sound conclusions. Therefore, data from the Maastricht Cohort study were used to investigate the longitudinal relation between smoking and shift work in a much larger population. In this study, a total of 12,140 employees were followed for two years by means of self-administered questionnaires. The authors compared workers who normally worked during daytime hours only (74%) with those who worked other than day shifts (26%). Logistic regression analyses were performed, adjusting for demographic factors of age, gender, and educational level to evaluate the risk of starting to smoke (n=25) in the group of non-smoking workers and the risk of quitting (n=318) in the group of smoking workers. Logistic regression analysis showed a significant association between shift work and taking up smoking during the two-year follow-up (odds ratio: 1.46, p=0.03). The risk to stop smoking was somewhat lower in shift workers (odds ratio: 0.91) but not statistically significant (p=0.5). To conclude, this study showed that, independent of educational level, shift workers are more prone to start smoking. This finding might have important implications for studies on the health effects of shift workers and for possible interventions aimed at the reduction of the excess health risk among shift workers.     

Methodological Aspects of Shift-Work Research

A major issue in shift-work research is to understand the possible ways in which shift work can impact performance and health. Nearly all body functions, from those of the cellular level to those of the entire body, are circadian rhythmic. Disturbances of these rhythms as well as the social consequences of odd work hours are of importance for the health and well-being of shift workers. This article reviews a number of common methodological issues which are of relevance to epidemiological studies in this area of research. It discusses conceptual problems regarding the use of the term “shift work,” and it underscores the need to develop models that explain the mechanisms of disease in shift workers.     

The Relation of Shift Work Tolerance to the Circadian Adjustment

The amplitude and phasing of circadian rhythms are under discussion as possible predictors of tolerance to night work. In a field study, subjective sleepiness and oral temperature of 147 female nurses were measured at 2-hour intervals during a period with one morning shift and two consecutive night shifts. The nurses also filled out a questionnaire. Two types of tolerance indices were constructed: The “health index” was based on questions referring to general fatigue, gastrointestinal symptoms, and sleep disturbances, and the “sleepiness index” on the actual subjective ratings of sleepiness. According to the health index, the group with good tolerance had a larger circadian amplitude of the oral temperature rhythm on the day of the morning shift than the group with poor tolerance. However, with regard to the sleepiness index, the corresponding difference between the groups with good or poor tolerance was not significant. The data did not confirm the hypothesis that predicts a quick adjustment of the circadian rhythm when the circadian amplitude is small before the change to night work. The contradictory results found in this and in other studies do not yet permit prediction of tolerance to night work.     

The Relation of Shift Work Tolerance to the Circadian Adjustment

The amplitude and phasing of circadian rhythms are under discussion as possible predictors of tolerance to night work. In a field study, subjective sleepiness and oral temperature of 147 female nurses were measured at 2-hour intervals during a period with one morning shift and two consecutive night shifts. The nurses also filled out a questionnaire. Two types of tolerance indices were constructed: The “health index” was based on questions referring to general fatigue, gastrointestinal symptoms, and sleep disturbances, and the “sleepiness index” on the actual subjective ratings of sleepiness. According to the health index, the group with good tolerance had a larger circadian amplitude of the oral temperature rhythm on the day of the morning shift than the group with poor tolerance. However, with regard to the sleepiness index, the corresponding difference between the groups with good or poor tolerance was not significant. The data did not confirm the hypothesis that predicts a quick adjustment of the circadian rhythm when the circadian amplitude is small before the change to night work. The contradictory results found in this and in other studies do not yet permit prediction of tolerance to night work.     

Sleep Disturbances Among Female Nurses: Comparing Shift to Day Work

Recently, attention has focused in Israel on the possible legal and health consequences of shift work. We decided to study sleep disorders among female nursing personnel working a shift schedule, in comparison with day nurses, in a large metropolitan general hospital. The study population was composed of 131 female certified nurses working shifts and 44 working days only. Inclusion criteria for the survey was at least 1 year of shift work alternating between day, evening, and night shifts, or at least 1 year of day work. All participants completed a self-report sleep questionnaire encompassing (a) demographic data, (b) sleep survey, and (c) employment details. Statistical analysis was performed using the Pearson correlation test and analysis of variance multiple range test (according to Scheffe's procedure). No significant correlation was found between sleep disorders and age of subjects. No sleep disorders were reported by 19.8% of shift workers versus 76.5% of day workers. Statistically significant findings were that the number of shifts per week >4.1 (p = 0.001) and duration of shift work >13.6 years (p = 0.007) correlated with the presence of sleep disorders. An additional significant finding (p = 0.014) was the impact of evening shifts on sleep disorders. The present small study confined to women supports the growing body of data on sleep complaints among shift workers.     

Nightshift work,chronotype, and genome-wide DNA methylation in blood

Molecular mechanisms underlying the negative health effects of shift work are poorly understood, which remains a barrier to developing intervention strategies to protect the long-term health of shift workers. We evaluated genome-wide differences in DNA methylation (measured in blood) between 111 actively employed female nightshift and 86 actively employed female dayshift workers from the Seattle metropolitan area. We also explored the effect of chronotype (i.e., measure of preference for activity earlier or later in the day) on DNA methylation among 110 of the female nightshift workers and an additional group of 131 male nightshift workers. Methylation data were generated using the Illumina Infinium HumanMethylation450 BeadChip (450K) Array. After applying the latest methylation data processing methods, we compared methylation levels at 361,210 CpG loci between the groups using linear regression models adjusted for potential confounders and applied the false-discovery rate (FDR) ≤ 0.05 to account for multiple comparisons. No statistically significant associations at the genome-wide level were observed with shift work or chronotype, though based on raw P values and absolute effect sizes, there were suggestive associations in genes that have been previously linked with cancer (e.g., BACH2, JRK, RPS6KA2) and type-2 diabetes (e.g., KCNQ1). Given that our study was underpowered to detect moderate effects, examining these suggestive results in well-powered independent studies or in pooled data sets may improve our understanding of the pathways underlying the negative health effects of shift work and the influence of personal factors such as chronotype. Such an approach may help identify potential interventions that can be used to protect the long-term health of shift workers.     

Late chronotype and high social jetlag are associated with burnout in evening-shift workers: Assessment using the Chinese-version MCTQshift

Chronotypes are associated with shift work tolerance and sleep in shift workers, and sleep mediates the impact of shift work on mental health. However, the role of chronotype in the association between shift work and mental health has not been clarified. In this study, we aimed to examine the association between chronotype and burnout in shift workers, using the validated Munich ChronoType Questionnaire for shift workers (MCTQ^shift). A total of 288 shift workers with irregular shift frequencies were recruited and completed the Chinese-version MCTQ^shift and the Morningness–Eveningness Questionnaire (MEQ). Chronotypes were assessed by the calculation of corrected mid-sleep time (MSF_SC) from mid-sleep time on free days (MSF) based on their exact shift schedules. Another 26 evening-shift nurses were monitored with actigraphy for at least two consecutive evening shifts and the following two free days. Burnout was evaluated using the Copenhagen Burnout Inventory. We found that MSF^E_SC, MSF^E and mid-sleep time on workdays (MSW^E) had normal distributions and correlated significantly with MEQ scores (r = ? 0.47, ?0.45 and ?0.47, respectively; all p < 0.001). MSW was more closely correlated with actigraphy-derived mid-sleep time on the free day before workdays than that on workdays (r = 0.61 and 0.48, respectively, p < 0.05). Sleep duration was significantly longer on workdays among evening-shift workers who slept late on workdays than those who slept early (β = 0.59, p < 0.001). After demographic and work characteristics were adjusted for in linear regression models, late chronotype and high social jetlag were associated with burnout scores in evening-shift workers. In conclusion, the Chinese-version MCTQ^shift is a valid tool for chronotype assessment. Interventions to improve sleep in shift workers should be tailored to chronotype due to variations in sleep behavior. Late chronotype may be an inherent feature of mental health problems, because the association with burnout was significant in both day workers in previous studies and shift workers.     

Differences in Sleep,Light, and Circadian Phase in Offshore 18:00–06:00 h and 19:00–07:00 h Shift Workers

Complaints concerning sleep are high among those who work night shifts; this is in part due to the disturbed relationship between circadian phase and the timing of the sleep‐wake cycle. Shift schedule, light exposure, and age are all known to affect adaptation to the night shift. This study investigated circadian phase, sleep, and light exposure in subjects working 18:00–06:00 h and 19:00–07:00 h schedules during summer (May–August). Ten men, aged 46±10 yrs (mean±SD), worked the 19:00–07:00 h shift schedule for two or three weeks offshore (58°N). Seven men, mean age 41±12 yrs, worked the 18:00–06:00 h shift schedule for two weeks offshore (61°N). Circadian phase was assessed by calculating the peak (acrophase) of the 6‐sulphatoxymelatonin rhythm measured by radioimmunoassay of sequential urine samples collected for 72 h at the end of the night shift. Objective sleep and light exposure were assessed by actigraphy and subjective sleep diaries. Subjects working 18:00–06:00 h had a 6‐sulphatoxymelatonin acrophase of 11.7±0.77 h (mean±SEM, decimal hours), whereas it was significantly later, 14.6±0.55 h (p=0.01), for adapted subjects working 19:00–07:00 h. Two subjects did not adapt to the 19:00–07:00 h night shift (6‐sulphatoxymelatonin acrophases being 4.3±0.22 and 5.3±0.29 h). Actigraphy analysis of sleep duration showed significant differences (p=0.03), with a mean sleep duration for those working 19:00–07:00 h of 5.71±0.31 h compared to those working 18:00–06:00 h whose mean sleep duration was 6.64±0.33 h. There was a trend to higher morning light exposure (p=0.07) in the 19:00–07:00 h group. Circadian phase was later (delayed on average by 3 h) and objective sleep was shorter with the 19:00–07:00 h than the 18:00–06:00 h shift schedule. In these offshore conditions in summer, the earlier shift start and end time appears to favor daytime sleep.     

Morningness or Morning Affect? A Short Composite Scale of Morningness

The Composite Scale of Morningness (CSM) is a widely used measure of behavioral temporal preference, and it is highly reliable across cultures. There are several competing models concerning its factor structure. We used confirmatory factor analysis (CFA) to replicate the two‐and three‐factor models, and, in addition, we examined the utility of the four‐item morning affect scale as a proxy for the full CSM. We tested these models in a large student sample (N=1396). The chi‐square result for the two‐ and three‐factor models indicated they did not adequately fit the data, and the modification indices suggested some items could be correlated. Allowing these items to correlate failed to produce a non‐significant chi‐square result, but some improvements to the incremental fit indices and root mean square error of approximation (RMSEA) were found. In particular, the three‐factor model seemed satisfactory. CFA of the morning‐affect scale also failed to fit the data, but the incremental fit indices and RMSEA were strong. For each of these three models, there was no significant difference between the invariant (no sex difference) and variant (sex difference) models. However, in each model, a critical ratio difference was found on the standardized regression weights between males and females on item 12. The morning‐affect scale had high reliability (0.83) and demonstrated satisfactory construct validity against the preferences scale and time to sleep and wake. In addition, high and low morning‐affect groups reported significant sleepiness differences by time of day. These results suggest the morning‐affect scale may be a useful proxy for the full scale. Furthermore, the items make no reference to time‐of‐day and may better facilitate cross‐cultural research. Studies using population‐based samples are recommended to further test the efficacy of the morning‐affect scale.     

Metabolic Syndrome in Permanent Night Workers

Night and shift work might be risk factors for metabolic and cardiovascular disorders due to interference with diet, circadian metabolic rhythms, and lifestyle. The relationship between permanent night work and metabolic and cardiovascular risk factors was explored in a retrospective longitudinal study of workers employed in a large municipal enterprise in charge of street cleaning and domestic waste collection. All subjects who had worked night shifts between 1976 and 2007 as hand sweepers, motor sweepers, and delivery tricar drivers were compared with subjects who always worked the same jobs but on day shifts. From the periodical medical surveillance files, we identified 488 male workers who had been examined on average five times (minimum 2, maximum 14) during the study period, for a total of 2,328 medical examinations; 157 always had worked day shifts, 12 always the night shift, and 319 both (initially day and subsequently night shifts). Their age ranged from 22 to 62 yrs, and work experience varied from 1 to 28 yrs. Lifestyle habits (smoking, alcohol consumption), body mass index, serum glucose, total cholesterol, tryglicerides, hepatic enzymes, blood pressure, resting electrocardiogram, diabetes, coronary heart disease, hypertension, and related drugs were taken into consideration for the analysis. We used generalized estimating equations (GEE) models (exchangeable correlation matrix) to analyze the relationship between night work and health effects while accounting for within‐subject correlations and adjusting for study period, job, age, and lifestyle variables. As a whole, night workers smoked more and had significantly higher BMI, serum total cholesterol, and triglycerides than day workers. Both the inter‐individual comparison between day and night workers and the intra‐individual comparison among the workers, who were day workers at the beginning of their employment and later became night workers, showed a significant increase in BMI, total cholesterol, and tryglicerides associated with night work. No consistent effect was seen on fasting glucose, hepatic enzymes, and blood pressure, whereas a higher incidence of coronary heart disease was recorded in night workers.     

A breed-matched case-control study of potential risk-factors for canine pyometra

The objective was to evaluate plausible risk-factors for pyometra, a common disease affecting almost 25% of all (unspayed) female dogs before 10 years of age. Because of the strong breed-predilection, an age- and breed-matched case-control study was undertaken on 87 pairs (pyometra-cases and healthy controls) from five breeds (Rottweiler (n = 13), Collie (n = 8), Golden retriever (n = 24), Labrador retriever (n = 16) and German shepherd dog (n = 26)). The mean age was 7.9 y (range 0.8-13.8 y). Variables analyzed included pseudopregnancy, age at first oestrus, length of and regularity of the interoestrus interval, hormone treatments, nulliparity, number of parities, age at first whelping, previous urinary tract infections and mammary tumours. Data were modelled multivariably using matched-pair conditional logistic regression. Analysing interactions with breed, previous pregnancy was statistically associated with pyometra. When amalgamated, in three breeds previous pregnancy was protective (Rottweiler, Collie, Labrador retriever) and in one breed statistically intermediate (German shepherd dog) when compared to the baseline (Golden retriever). Previous pregnancy was a statistically significant factor that had a protective effect against pyometra in some breeds but not in the Golden retriever breed. These findings indicate that protective- and risk-factors may vary between different breeds. The obvious problem with low power and limited possibility for extrapolation, using few dogs in few breeds, is acknowledged. However, it is suggested that failure to control for the confounding effect of breed, especially in epidemiological studies on dog diseases, may lead to potentially erroneous conclusions.     

Measurement of,and Some Reasons for,Differences in Eating Habits Between Night and Day Workers

A questionnaire was designed to assess the following: why working people chose to eat or not to eat at a particular time of day; the factors that influenced the type of food eaten; and subjective responses to the meal (hunger before, enjoyment during, satiety afterward). Self-assessments were done every 3h during a typical week containing work and rest days, by one group of 50 day workers and another group of 43 night workers. During the night work hours compared to rest days, night workers evidenced a significantly altered food intake, with a greater frequency of cold rather than hot food (p < 0.001). The type and frequency of meals were influenced significantly more (p < 0.05) by habit and time availability and less by appetite. This pattern continued into the hours immediately after the night shift had ended. In day workers food intake during work hours, compared to rest days, was also influenced significantly more often (p < 0.05) by time availability than hunger, but less so than with night workers. Moreover, day workers were less dependent than night workers upon snacks (p = 0.01), and any significant differences from rest days did not continue beyond work hours. Not only did night workers change their eating habits during work days more than did day workers but also they looked forward to their meals significantly less (p < 0.001) and felt more bloated after consuming them (p < 0.05), such effects being present to some extent during their rest days also. These findings have clear implications for measures designed to ease eating problems that are commonly problematic in night workers.     

Impact of night-float rotation on sleep,mood, and alertness: the resident's perception

Night-float rotations were designed to alleviate the workload of residents on night call and thereby improve patient safety. However, the impact of the night float on residents is yet to be surveyed. We assessed the impact of the night-float rotation on pediatric residents using an anonymous questionnaire that covered topics, based on recall, about sleep, mood, alertness, adjustment, and others. The study was conducted in a major tertiary pediatric teaching hospital in the United States. Participants were pediatric residents who had completed one or two night-float rotations and were in active training at our teaching hospital at the time of the study. Fifty-two of 60 eligible residents (87%) responded. Sleep duration during the night-float rotation was shorter than during day-shift work in 24 residents (46%), longer in 20 (38%), and unchanged in eight (15%). A higher proportion of residents took longer to fall asleep, had more difficulty falling asleep, had more sleep interruptions, and felt less rested upon awakening. Twenty-four residents (46%) felt that their bodies never adjusted to the night shift. Also, 22 residents (43%) felt moody or depressed in contrast to seven (14%) who felt depressed during the daytime rotation (p=0.0001). Twenty-one residents (41%) felt they were slower in their thinking during the night float than daytime rotations. The results suggest that disturbances of sleep and mood and decreased alertness, typical of night shift, are present in the night-float rotation. Residency programs should monitor closely the impact of the night-float rotation on resident well being and patient safety. The impact of night-shift work should be considered in the design of night-float schedules, and teaching should be provided for residents to learn coping strategies for night-shift work.     

Euchronism,Allochronism, and Dyschronism: Is Internal Desynchronization of Human Circadian Rhythms a Sign of Illness?

The authors define a subject as euchronic when the circadian parameters—tau (τ=period), Ø (acrophse or peak time), A (amplitude), and M (MESOR=24 h rhythm‐adjusted mean)—of a set of circadian variables are within the confidence limits of appropriate reference values of healthy subjects (HS). We define internal desynchronization as a state in which the circadian τ of a set of rhythms differs from 24 h and when the τ of a given variable differs from that of other variables. Such a state was first observed in singly isolated HS without access to time cues and clues. Herein, data and analyses are presented demonstrating that internal desynchronization appears to be a rather common phenomenon in HS dwelling in their natural environment (i.e., in the presence of usual zeitgebers). This has been documented by longitudinal studies (n?15 days) of the circadian rhythm in sleep‐wakefulness, body temperature, right‐ and left‐hand‐grip strength, and reaction time involving a total of 246 HS and 134 shift workers (SW), with 45.5% showing good and 54.5% poor SW tolerance. The presence of internal desynchronization observed in SW was associated SW intolerance, with symptoms being sleep alteration/disturbances, sleeping‐pill dependence, persisting fatigue (asthenia), mood alteration, and digestive complaints. Internal desynchronization was also documented in groups of HS and tolerant SW, though it was almost the rule among the intolerant SW. The authors introduce two new terms: allochronism to describe the time organization of those SW who evidence internal desynchronization without detectable clinical symptoms, and dyschronism to describe the time organization of those SW who exhibit internal desynchrobization plus the symptoms of SW intolerance or medical illness. The condition of allochronism is not restricted only to SW tolerance, as it was detected in 112 HS without medical complains when exposed to various experimental conditions, including medications and placebos, sojourn in the high Arctic summer, intensive sport training, and task‐loaded cognitive performance testing. Dyschronism in SW who are sleep‐deprived is associated with persisting fatigue. An unpublished Gallup survey found that 47% of 2478 respondents experienced a state of asthenia during the previous 12 months, with symptoms mimicking those of SW intolerance. In one‐third of the cases, the origin of the asthenia was undetermined. Taking into account the high incidence of internal desynchronization found in past investigations and the clinical observation that sleep deprivation is a consequence of many acute and chronic medical conditions (nocturnal pain, nocturnal asthma, etc.), it is suggested that dyschronism may be responsible for the asthenia of unknown origin, at least for some persons. The interindividual (including sex‐related) variability in the propensity to exhibit an altered temporal organization, whether it be transient or persistent (i.e., reversible or non‐reversible) suggests the involvement of genetic factors. The Dian‐Circadian genetic model previously proposed by the authors seems pertinent to conceptualize and explain the various levels and output of internal desynchronization.     

IMPACT OF NIGHT-FLOAT ROTATION ON SLEEP,MOOD, AND ALERTNESS: THE RESIDENT'S PERCEPTION

Night-float rotations were designed to alleviate the workload of residents on night call and thereby improve patient safety. However, the impact of the night float on residents is yet to be surveyed. We assessed the impact of the night-float rotation on pediatric residents using an anonymous questionnaire that covered topics, based on recall, about sleep, mood, alertness, adjustment, and others. The study was conducted in a major tertiary pediatric teaching hospital in the United States. Participants were pediatric residents who had completed one or two night-float rotations and were in active training at our teaching hospital at the time of the study. Fifty-two of 60 eligible residents (87%) responded. Sleep duration during the night-float rotation was shorter than during day-shift work in 24 residents (46%), longer in 20 (38%), and unchanged in eight (15%). A higher proportion of residents took longer to fall asleep, had more difficulty falling asleep, had more sleep interruptions, and felt less rested upon awakening. Twenty-four residents (46%) felt that their bodies never adjusted to the night shift. Also, 22 residents (43%) felt moody or depressed in contrast to seven (14%) who felt depressed during the daytime rotation (p=0.0001). Twenty-one residents (41%) felt they were slower in their thinking during the night float than daytime rotations. The results suggest that disturbances of sleep and mood and decreased alertness, typical of night shift, are present in the night-float rotation. Residency programs should monitor closely the impact of the night-float rotation on resident well being and patient safety. The impact of night-shift work should be considered in the design of night-float schedules, and teaching should be provided for residents to learn coping strategies for night-shift work.