Shiftwork and cardiovascular disease: review of disease mechanisms

The triad of shift work, occupational noise, and physical workload and risk of coronary heart disease

BACKGROUND

Shift work, noise, and physical workload are very common occupational exposures and they tend to cluster in the same groups of workers.

OBJECTIVES

To study the short and long term effects of these exposures on risk of coronary heart disease (CHD) and to estimate the joint effects of these factors.

METHODS

The study population in this prospective 13 year follow up study of 1804 middle aged industrially employed men was collected at the first screening for the Helsinki Heart Study. The CHD end points (ICD-9 codes 410-414 and ICD-10 codes I20-I25) were obtained from official Finnish registers. The Finnish job-exposure matrix FINJEM provided information on occupational exposures. Relative risks (RR) of CHD for the exposures were estimated using Cox's proportional hazard models adjusting for classical risk factors of CHD.

RESULTS

The RR in the five year follow up for continuous noise combined with impulse noise was 1.28; for shift work it was 1.59, and for physical workload 1.18, while in the 13 year follow up the RRs were 1.58, 1.34, and 1.31, respectively. When adjusted for white-collar/blue-collar status the RRs decreased markedly. The RR in the 13 year follow up for those exposed to two risk factors was close to 1.7 and for those exposed to all three, 1.87.

CONCLUSION

Shift work and continuous noise entailed an excess risk for CHD in the shortest follow up with only a few retired workers but a decreasing risk during the longer follow up. For physical workload and impulse noise the trend was opposite: the CHD risk was increasing with increasing follow up time despite increasing numbers of retired workers.

The Use of Chronobiotics in the Resynchronization of the Sleep–wake Cycle

Treatment of circadian rhythm disorders, whether precipitated by intrinsic factors (e.g., sleep disorders, blindness, mental disorders, aging) or by extrinsic factors (e.g., shift work, jet-lag) has led to the development of a new type of agents called 'chronobiotics', among which melatonin is the prototype. The term 'chronobiotic' defines as a substance capable of shifting the phase of the circadian time system thus re-entraining circadian rhythms. Melatonin administration synchronizes the sleep-wake cycle in blind people and in individuals suffering from delayed sleep phase syndrome or jet lag, as well in shift-workers. The effect of melatonin on sleep is probably the consequence of increasing sleep propensity (by inducing a fall in body temperature) and of a synchronizing effect on the circadian clock (chronobiotic effect). We successfully employed the timely use of three factors (melatonin treatment, exposure to light, physical exercise) to hasten the resynchronization after transmeridian flights comprising 12-13 time zones, from an average of 8-10 days to about 2 days. Daily melatonin production decreases with age, and in several pathologies, attaining its lowest values in Alzheimer's dementia patients. About 45% of dementia patients have severe disruptions in their sleep-wakefulness cycle. Both in aged subjects having very minimal sleep disorders as well as in demented patients with a very severe disorganization of the sleep-wake cycle, melatonin treatment reduced the variability of sleep onset and restored sleep.

Long-Term Longitudinal Study on the Relationship Between Alternating Shift Work and the Onset of Diabetes Mellitus in Male Japanese Workers

OBJECTIVE

This study investigated the effect of alternating shift work (ASW) on the onset of diabetes mellitus in Japanese workers compared with onset in day-shift work (DSW).

METHODS

A longitudinal study was carried out on a DSW group (n = 3203) and ASW group (n = 2426) of a steel company who received their annual health checkups over a 10-year period between 1991 and 2001. The association between job schedule type and onset of diabetes mellitus (glycated hemoglobin A1c > or =6.0% or medication) was investigated by multivariate pooled logistic regression analyses.

RESULTS

The odds ratio (95% confidence interval) for the development of diabetes mellitus in the ASW group compared with the DSW group was 1.35 (1.05-1.75).

CONCLUSIONS

Our study revealed that the ASW is an independent risk factor for the onset of diabetes mellitus.

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.

Shiftwork and metabolic risk factors of cardiovascular disease

We conducted this study to examine the relationship between shift work duration and the metabolic risk factors of cardiovascular disease among shift workers. The study population consisted of 226 female hospital nurses and 134 male workers at a firm manufacturing diapers and feminine hygiene materials, whose mean ages were 28.5 yr for the nurses and 29.1 yr for the male workers. The fasting blood sugar level, serum cholesterol, blood pressure, height and weight, waist and hip circumferences (only for the nurses), and numbers of walks during work (as a measure of physical activity) were measured. Using the Karasek's job contents questionnaire, job stress was assessed. Information about the years of work, shift work duration, past medical and behavioral history, including smoking, was obtained by a self-administered questionnaire. With definitions of hypertension as systolic blood pressure (SBP) > or =160 or diastolic blood pressure (DBP) > or =90 mmHg occurring at least once, hypercholesterolemia as serum total cholesterol > or =240 mg/dl, obesity as body mass index (BMI) > or =25 kg/m(2) and as waist to hip ratio (WHR) > or =0.85, we examined the prevalences of metabolic risk factors among subjects. Regression analyses to show the relationships between shift work duration and metabolic risk factors were performed using simple and multivariate models stratified by age, and adjusted for smoking, drinking, job strain and physical activity. Duration of shift work was significantly associated with SBP or cholesterol level among male workers aged 30 or more. Among female nurses, it was inversely associated with DBP (in those who were below 30 yr old) and cholesterol (in those who were aged 30 or more). BMI was non-significantly associated with the duration of shift work in both male workers and female nurses who were 30 yr old or more. WHR in female nurses increased slightly according to increasing duration of shift work. Fasting blood sugar was not significantly associated with the duration of shift work in either sex regardless of age-group. These results suggest an association between shift work duration and the metabolic risk factors of cardiovascular disease.

Daily rhythms in plasma levels of homocysteine

BACKGROUND: There is accumulated evidence that plasma concentration of the sulfur-containing amino-acid homocysteine (Hcy) is a prognostic marker for cardiovascular morbidity and mortality. Both fasting levels of Hcy and post methionine loading levels are used as prognostic markers. The aim of the present study was to investigate the existence of a daily rhythm in plasma Hcy under strictly controlled nutritional and sleep-wake conditions. We also investigated if the time during which methionine loading is performed, i.e., morning or evening, had a different effect on the resultant plasma Hcy concentration. METHODS: Six healthy men aged 23-26 years participated in 4 experiments. In the first and second experiments, the daily rhythm in Hcy as well as in other amino acids was investigated under a normal or an inverse sleep-wake cycle. In the third and fourth, Hcy concentrations were investigated after a morning and evening methionine loading. To standardize food consumption in the first two experiments, subjects received every 3 hours 150 ml of specially designed low-protein liquid food (Ensure(R) formula). RESULTS: In both the first and second experiments there was a significant daily rhythm in Hcy concentrations with a mid-day nadir and a nocturnal peak. Strikingly different 24-h patterns were observed in methionine, leucine, isoleucine and tyrosine. In all, the 24-h curves revealed a strong influence of both the sleep-wake cycle and the feeding schedule. Methionine loading resulted in increased plasma Hcy levels during both morning and evening experiments, which were not significantly different from each other. CONCLUSIONS: There is a daily rhythm in plasma concentration of the amino acid Hcy, and this rhythm is independent of sleep-wake and food consumption. In view of the fact that increased Hcy concentrations may be associated with increased cardiovascular risks, these findings may have clinical implications for the health of rotating shift workers.

Gender differences in sleep and sleep-disordered breathing

Sleep and sleep disorders are different in several important ways between men and women. Because of pregnancy and menopause, women experience changes in sleep that may present as clinical problems. In clinical populations, women are more likely to present with insomnia than are men, although their sleep may be better preserved. The presentation of sleep apnea in women is distinct from that of men and is less likely to include a "classic" history of witnessed ap-nea or heavy snoring. More likely it presents with nonspecific symptoms, such as fatigue or mood disturbance. There are little data on the effects of different treatments for OSA between men and women. OHS is a syndrome that may be as common in women as in men. The role of hormones in its pathophysiology is not well-defined.

Impact of One Year of Shift Work on Cardiovascular Disease Risk Factors

The purpose of the study was to investigate whether the reported increased cardiovascular disease risk in shift workers could be explained by changes in cardiovascular risk factors. In a cohort of 239 shift and 157 daytime workers, 1-year changes in biological and lifestyle cardiovascular risk factors were monitored between the start of a new job and 1 year later. Both body mass index and low-density lipoprotein/high-density lipoprotein cholesterol ratio decreased significantly in shift workers compared with daytime workers (body mass index change: -0.31 and +0.13 kg/m; low-density lipoprotein/high-density lipoprotein ratio change: -0.33 and -0.13 respectively). Cigarettes smoked per day increased significantly in shift compared with daytime workers (+1.42 and -1.03, respectively). Therefore, only for smoking, an unfavorable change was observed. This may explain, at most, only a part of the excess cardiovascular disease risk reported in shift workers.

Subjective and Objective Evaluation of Sleep and Performance in Daytime Versus Nighttime Sleep in Extended-Hours Shift-Workers at an Underground Mine

Extended hours of shift work has the potential for adverse consequences for workers, particularly during the nightshift, such as poorer sleep quality during the day, increased worker fatigue, and fatigue-related accidents and decreased work performance. This study examined subjective and objective measurements of sleep and performance in a group of underground miners before and after the change from a backward-rotating 8-hour to a forward-rotating 10-hour shift schedule. The purpose of this study was to evaluate the short- and long-term impact of a shift schedule change on sleep and performance. The results demonstrated improved subjective and objective measures of sleep and performance on the new 10-hour nightshift schedule. The 10-hour nightshift workers subjectively reported more refreshing sleep, fewer performance impairments and driving difficulties than 8-hour nightshift workers. The results of the objective measures of sleep and performance on the 10-hour nightshifts were overall similar or possibly better than those measured on the 10-hour dayshifts. These are some of the first data to suggest that a nightshift that does not encompass the entire night period could have significant benefits to shift-workers. We suggest that these benefits are mostly the result of the timing of the new nightshift start and end times rather than other shift-schedule factors.

Shiftwork and environment as interactive predictors of work perceptions

Evidence suggests that the work environment may play a role in the elevated risk of adverse health outcomes among shiftworkers compared with dayworkers. Perceived work environment measures (physical stressors, job demand, job control, skill discretion, supervisor support, and safety perceptions) from UK oil industry personnel (N=1,867) were analyzed in relation to shiftwork (day/night rotation vs daywork) and objective work environment (onshore vs offshore). Age, education, job type, and negative affectivity were also included. The Environment * Shiftwork interaction was significant in multivariate tests and in 5 of the 6 univariate analyses. Onshore, shiftworkers perceived their environment significantly less favorably than dayworkers, but differences were less marked offshore. Results are discussed in relation to the demand-control-support model of work stress.

Metabolic disturbances in male workers with rotating three-shift work. Results of the WOLF study

OBJECTIVES

The aim of the present study was to investigate the relationship between important metabolic risk factors for coronary heart disease (CHD) and type 2 diabetes in shift workers and day workers.

METHODS

Cross-sectional data from a sub-population in the WOLF study consisting of 665 day workers and 659 three-shift workers in two plants were analysed.

RESULTS

A higher proportion of shift workers than day workers had high triglyceride levels (> or =1.7 mmol/l), low levels of HDL-cholesterol (<0.9 mmol/l) and abdominal obesity (waist/hip ratio>0.9). The risk of low HDL-cholesterol was doubled in shift workers, (odds ratio (OR): 2.02, 95% confidence interval (95% CI): 1.24-3.28) after being adjusted for age, socio-economic factors, physical activity, current smoking, social support and job strain. High levels of triglycerides were also significantly associated with shift work (OR: 1.40, 95% CI: 1.08-1.83). The OR for abdominal obesity was 1.19, (95% CI: 0.92-1.56). The prevalence of hyperglycaemia (serum glucose > or =7.0 mmol/l) was similar in day and shift workers. No significant interaction was seen between shift work and abdominal obesity with regard to the associations with triglycerides and HDL-cholesterol.

CONCLUSIONS

We found a significant association between shift work and lipid disturbances (i.e. low HDL-cholesterol and high triglyceride levels). We did not find any association with hyperglycaemia.

The impact of roster changes on absenteeism and incident frequency in an Australian coal mine

BACKGROUND

The occupational health and safety implications associated with compressed and extended work periods have not been fully explored in the mining sector.

AIMS

To examine the impact on employee health and safety of changes to the roster system in an Australian coal mine.

METHODS

Absenteeism and incident frequency rate data were collected over a 33 month period that covered three different roster schedules. Period 1 covered the original 8-hour/7-day roster. Period 2 covered a 12-month period under a 12-hour/7-day schedule, and period 3 covered a 12-month period during which a roster that scheduled shifts only on weekdays, with uncapped overtime on weekends and days off (12-hour/5-day) was in place. Data were collected and analysed from the maintenance, mining, and coal preparation plant (CPP) sectors.

RESULTS

The only significant change in absenteeism rates was an increase in the maintenance sector in the third data collection period. Absenteeism rates in the mining and CPP sectors were not different between data collection periods. The increase in the maintenance sector may be owing to: (1) a greater requirement for maintenance employees to perform overtime as a result of the roster change compared to other employee groups; or (2) greater monotony associated with extended work periods for maintenance employees compared to others. After the first roster change, accident incident frequency decreased in the CPP sector but not in the other sectors. There was no effect on incident frequency after the second roster change in any sector.

CONCLUSIONS

The current study did not find significant negative effects of a 12-hour pattern, when compared to an 8-hour system. However, when unregulated and excessive overtime was introduced as part of the 12-hour/5-day roster, absenteeism rates were increased in the maintenance sector. The combination of excessive work hours and lack of consultation with employees regarding the second change may have contributed to the overall negative effects.

[Social and professional factors, occupational environmental strain and cardiovascular diseases]

In addition to conventional risk factors, environmental and occupational strain is an actor of the development and evolution of cardiovascular diseases. In industrialised countries, cardiovascular mortality is inversely correlated with the socio-economic level and type of occupation. In the French Ihpaf study, systemic hypertension and obesity were correlated with the socio-economic level. Among possible explanations for the importance of occupational environment, psychological stress at work, sedentary jobs, passive smoking and shift working may all play a role. Thus, beyond the conventional approach to individual risk factor management, it appears necessary to consider cardiovascular prevention through collective actions taking into account occupational environment.

Is there an association between shift work and having a metabolic syndrome? Results from a population based study of 27,485 people

OBJECTIVES

To explore how metabolic risk factors for cardiovascular disease (CVD) differ between shift workers and day workers in a defined population. Shift work has been associated with an increased risk of CVD. Risk factors and causal pathways for this association are only partly known.

METHODS

A working population of 27,485 people from the Västerbotten intervention program (VIP) has been analysed. Cross sectional data, including blood sampling and questionnaires were collected in a health survey.

RESULTS

Obesity was more prevalent among shift workers in all age strata of women, but only in two out of four age groups in men. Increased triglycerides (>1.7 mmol/l) were more common among two age groups of shift working women but not among men. Low concentrations of high density lipoprotein (HDL) cholesterol (men<0.9 and women<1.0 mmol/l) were present in the youngest age group of shift workers in both men and women. Impaired glucose tolerance was more often found among 60 year old women shift workers. Obesity and high triglycerides persisted as risk factors in shift working men and women after adjusting for age and socioeconomic factors, with an OR of 1.4 for obesity and 1.1 for high triglyceride concentrations. The relative risks for women working shifts versus days with one, two, and three metabolic variables were 1.06, 1.20, and 1.71, respectively. The corresponding relative risks for men were 0.99, 1.30, and 1.63, respectively.

CONCLUSIONS

In this study, obesity, high triglycerides, and low concentrations of HDL cholesterol seem to cluster together more often in shift workers than in day workers, which might indicate an association between shift work and the metabolic syndrome.