Circadian excretory rhythms in night workers

Conroy, R. T. W. L., Elliott, A. L., and Mills, J. N. (1970).Brit. J. industr. Med.,27, 356-363. Circadian excretory rhythms in night workers. Urine samples have been collected from day and night shift workers in a light engineering factory over periods of 24 or 48 hours during which blood was also collected for determination of plasma 11-hydroxycorticosteroids (11-OHCS). Different subjects divided their 24-hour output into as few as 4 or as many as 14 portions. Where possible, the timing of the excretory rhythms was assessed objectively by fitting a sine curve. The potassium excretory rhythm was, in most subjects, well adapted to night work in that they excreted least during their hours of sleep; no relationship could be discerned between potassium excretory and plasma 11-OHCS rhythms. The sodium excretory rhythm was less regular than that of potassium in both night and day workers, and in night workers both sodium excretion and urine flow were often high during the hours of sleep, so that the subjects were sometimes awakened by the need to micturate. There was, however, sufficient association between the behaviour of sodium and potassium to suggest that their excretory rhythms have a common cause, and that this is not the secretion of 11-OHCS. Phosphate excretion fell sometimes on rising, even though plasma 11-OHCS concentration was not high, and sometimes after a high level of plasma 11-OHCS, at a time other than on rising. This suggests that these are two independent contributory causes of the usual morning fall in phosphate excretion.     

Effects of night work on urinary excretion rates of 6-sulfatoxymelatonin, norepinephrine and estriol in pregnant women

It has been suggested that shift work, night work in particular, affects worker's psychophysical health. However, the effects of night work on the health of pregnant women are not physiologically well elucidated. The effects of night work on the biological function of pregnant women were studied in the present study. Three pregnant and six non-pregnant nurses that engaged in fast-rotating shift system cooperated for the study. The 24-h urine samples were collected in two time frames, daytime (07:00 to 23:00) and nighttime (23:00 to 07:00) on the day shift, the night shift and the days off. Urinary concentrations of 6-sulfatoxymelatonin and norepinephrine were measured by enzyme-linked immunosorbent assay and estriol by radioimmunoassay. The excretion profiles of urinary 6-sulfatoxymelatonin in the pregnant and the non-pregnant women were altered on the night shift, which might relate the derangement of circadian rhythm of melatonin secretion. The urinary norepinephrine level during the night work was considerably higher in the pregnant women, which indicated the presence of higher physical burden in them due to the night work. The urinary estriol level was not significantly affected by shift, day/night and individual factors, and the effect of night work on urinary estriol level of the pregnant women remained uncertain. The results of the present study suggested that the night work of the pregnant shift workers should be much more relieved.     

Circadian rhythms of body temperature in shift-workers at a coalface

Colquhoun, W. P., and Edwards, R. S. (1970).Brit. J. industr. Med.,27, 266-272. Circadian rhythms of body temperature in shift-workers at a coalface. The oral temperatures of a group of coalface workers were recorded daily at intervals during their period of work underground, over a three-week cycle of day, afternoon and night shifts. The basic mean circadian rhythm of temperature observed in the group as a whole was similar in phase and in amplitude to that of a control group of young men unused to shift work. However, it appeared that the rhythm of older coalface workers differed to some extent from that of their younger colleagues, since the mean temperature in the second half of the night was significantly higher in the older men. Whether this difference is permanent, and whether it is due to age alone rather than to long experience of night-shift working, has yet to be determined.

There were indications that the changes in the mean on-shift trend in temperature during the week in response to successive days of an altered sleep-waking routine were more pronounced, in the group as a whole, than the changes observed in a control group of young men when the latter were exposed to similar unusual routines.

It is recommended that these findings be substantiated on a considerably larger number of coalface workers before their implications for shift working are considered.

     

Changes in melatonin and sex steroid hormone production among men as a result of rotating night shift work – the HORMONIT study

ObjectiveData from real world settings on circadian disruption and subsequent hormone-related changes may explain the higher risk of hormone-dependent cancers among night shift workers.The present study examines the melatonin and sex steroid hormone levels among night shift workers.MethodsWe included 44 male, rotating shift workers from a car factory in Spain, sampled both at the end of a 3-week night shift (22:00–06:00 hrs) and a 3-week early morning shift (06:00–14:00 hrs). Participants collected all urine voids over 24-hours during each shift. Urinary concentrations of sex steroid hormones (estrogens, androgens and progestogens) and 6-sulfatoxymelatonin (aMT6s, major melatonin metabolite) were determined. Individual cosinor analysis was used to derive the acrophase (peak time) and area under the curve (total production). Linear mixed models examined intraindividual associations between night shift work and log-transformed 24-hour peak time and total production of hormones compared to early morning shift work.ResultsThe acrophase was delayed during the night shift for aMT6s [geometric mean difference (GMD) 7.53 hrs, 95% confidence interval (CI) 4.46–10.60], androgens (eg, testosterone: GMD 6.83 hrs, 95% CI 0.34–13.32) and progestogens (eg, 17-hydroxyprogesterone: GMD 4.54 hrs, 95% CI 2.92–6.16) compared to the early morning shift. We found a higher production of adrenal androgen 11-oxoandrosterone/11-oxoetiocholanolone [geometric mean ratio (GMR) 1.43, 95% CI 1.12–1.81], and a lower production of adrenal progestogen 16-cysteinylprogesterone (GMR 0.79, 95% CI 0.67–0.93) during the night shift compared to the early morning shift levels.ConclusionsNight shift work was associated with melatonin and sex hormone-related changes in timing and total production, providing insight into the mechanistic path for its association with hormone-dependent cancer.     

Effects of consecutive night shifts with prolonged on-duty and overtime work on various sleep

The purpose of this research was to investigate the effect of consecutive night shifts upon various types of sleep. The subjects studied were 9 guards who usually worked on an alternate-day shift system. The nighttime nap EEG (3:00-6:30) during the 24-hr on-duty period was recorded, and then the subsequent day sleep EEG (11:00 to spontaneous awakening) and recovery night sleep (23:00 to spontaneous awakening) were also measured. These records were made for the nap, day sleep and recovery night under two different sets of conditions. The first was for the usual shift cycle (U), i.e. 24 hr duty (10:00-10:00)----off duty----24 hr duty----off duty. The other was for consecutive night shifts (CN) with overtime work (18:00-10:00) performed on the normal off day, i.e. 24 hr duty----8 hr off duty----40 hr duty----off duty. The results were as follows: There was no difference between U and CN conditions concerning time in bed and total sleep time in the nighttime nap, day sleep and recovery night. However, the total time of daytime sleep was significantly shortened under both U and CN conditions, as compared with the control night. The amount of REM sleep in the CN condition for both the nighttime nap and day sleep was significantly greater than in the U condition. In the recovery night, REM sleep continued to be slightly greater for the CN condition than for the U condition. Slow wave sleep (SWS) in the CN condition was slightly greater than in the U condition for the nighttime nap and day sleep, but in the recovery night, no difference was observed between the two conditions. In the U condition, the sleep onset REM period (SOREMP) was observed in 2 of the 9 cases, but only during the day sleep. In the CN condition, however, it occurred in 5 subjects during the nighttime nap, in 4 during the day sleep, and in 2 during the recovery night. The REM sleep latency in the CN condition thus tended to be diminished in each type of sleep compared with that in the U condition, but a significant difference between the two conditions was observed only in the nighttime nap. It is believed that the increase in the amount of REM sleep and in the frequency of SOREMP might be due to the repeated pattern of sleep loss and shortened periods of sleep during consecutive night shifts of prolonged duty, with partial sleep deprivation resulting from lack of REM sleep.     

Activity patterns associated with depression symptoms in retired day and night shift workers

ObjectiveEvidence regarding which activity patterns relate to depression symptoms can guide the development of approaches to prevent major depression and related consequences. While prior studies have linked activity pattern disruption and depression symptoms, little is known regarding these relationships in people who were previously exposed to the circadian and sleep-wake challenge of night shift work. We therefore examined: (1) if retired night shift workers (NSWs) and retired day shift workers (DSWs) had different activity pattern characteristics; and (2) which activity pattern characteristics correlate with depression symptoms in these groups.MethodsWe examined cross-sectional associations of activity patterns and depression symptoms in 64 retired NSWs (mean age = 68, standard deviation (SD) = 5) and 75 DSWs (mean age = 69, SD = 6). We used wrist actigraphy (mean days = 6.5, SD = 1.1) to measure activity levels and assessed activity patterns using non-parametric indices; and assessed depression symptoms as Center for Epidemiologic Studies-Depression Scale scores ≥8.ResultsRetired NSWs and retired DSWs did not differ in activity characteristics. Depression symptoms were associated with having less activity between 13:00 h and 17:00 h in retired NSWs (Cohen's D = 1.22) but not retired DSWs (Cohen's D = 0.20).ConclusionActivity patterns of prior NSWs may be comparable to those of prior DSW in retirement. However, uniquely in NSWs, we found that less afternoon activity was associated with depression symptoms. If persistent afternoon inactivity is confirmed as a predictor of depression risk in past NSWs, interventions targeting afternoon inactivity could be developed to prevent depression and its consequences in retired NSWs.     

The effects of bright light treatment on subjective and objective sleepiness during three consecutive night shifts among hospital nurses – a counter-balanced placebo-controlled crossover study

Objectives:The objective was to investigate effects of timed bright light treatment on subjective and objective measures of sleepiness during three consecutive night shifts among hospital nurses.Methods:Thirty-five nurses were exposed to bright light (10,000 lux) and red dim light (100 lux) during three consecutive night shifts in a counter-balanced crossover trial lasting nine days, which included three days before and three days after the three night shifts. Light exposure for 30 minutes was scheduled between 02:00–03:00 hours on night 1, and thereafter delayed by one hour per night in order to delay the circadian rhythm. Subjective sleepiness was measured daily (heavy eyelids, reduced performance) and every second hour while awake (Karolinska Sleepiness Scale, KSS). Objective sleepiness (Psychomotor Vigilance Task, PVT) was measured at 05:00 hours during each night shift. Beyond nocturnal light exposure on the night shifts, no behavioral restrictions or recommendations were given at or off work.Results:Bright light treatment significantly reduced heavy eyelids during night shifts. However, results on KSS and PVT were unaffected by bright light. There were no differences in subjective sleepiness during the three days following the night shifts.Conclusions:This bright light treatment protocol did not convincingly reduce sleepiness among nurses during three consecutive night shifts. Nor did bright light impede the readaptation back to a day-oriented rhythm following the night shift period. Too few consecutive night shifts, inappropriate timing of light, and possible use of other countermeasures are among the explanations for the limited effects of bright light in the present study.     

Neuroendocrine recovery after 2-week 12-h day and night shifts: an 11-day follow-up

Purpose

The study aimed to investigate the course and duration of neuroendocrine recovery after 2-week 12-h day and night shift working periods and to study whether there were differences in recovery between the shift groups.

Methods

Twenty-nine male offshore employees working 2-week 12-h shift tours participated in the study; 15 participated after a day shift tour and 14 after a night shift tour. Salivary cortisol was assessed at awakening, 30 min after awakening, and before bedtime on the 1st, 4th, 7th, and 11th day of the free period, with a reference day prior to the offshore tour. Differences were tested using generalised estimating equations analysis.

Results

Compared to the reference day, night shift workers had a significantly flatter cortisol profile on the 1st day off, significantly lower cortisol concentrations at 30 min after awakening on day 4 and at awakening on day 7, and a significantly smaller decline to evening concentration on days 4 and 11. Compared to the reference day, day shift workers only showed a significantly lower cortisol concentration at awakening on the 1st day off. Compared to day workers, night shift workers had a flatter profile on the 1st day off and a lower cortisol concentration at awakening on the 4th day.

Conclusions

Following 2-week 12-h night shift working periods, recovery was not fully complete up to day 11. Following 2-week 12-h day shift working periods, an indication of incomplete recovery was found on the 1st day off, with full recovery reached on day 4.

     

An occupational health physician's report on the improvement in the sleeping conditions of night shift workers

This paper reviews briefly our research findings on sleep and health for shift workers at a bakery and a dish factory and aims to give some information on health protection with improved sleep. Our medical examinations revealed that rises in blood pressure (BP) were frequently observed in male bakery workers on the fixed night shift. They took a two-hour nap from 1:00 to 3:00 during the night shifts according to our advice. As a result, their diastolic BP significantly decreased in subsequent years. Women working shifts before 3:00 at a dish factory seemed to show more sleep disturbance, higher daytime sleepiness, and more fatigue than those working shits from 3:00. Total sleep hours taken during the daytime was similar between two groups. Among the dish, factory workers who switched to midnight or night shifts, 70% reported slow recovery from fatigue and 30% felt deteriorated health. These women also reported increased efforts to obtain a deep sleep, probably making up for shortened sleep. Occupational physicians can act to improve shift work conditions through monitoring the health of workers, applying naps, and designing possible schedules to secure deep, sufficient sleep.     

Absenteeism of shift and day workers: A study of six types of shift system in 29 organizations

Taylor, P. J., Pocock, S. J., and Sergean, R. (1972).Brit. J. industr. Med.,29, 208-213. Absenteeism of shift and day workers. Previous evidence on the effects of shift work upon absence behaviour is conflicting, this being due in part to the variety of shift systems in use. A study is described in which absence records over two years were obtained for 965 matched pairs of shift and day workers from 29 organizations. Six types of shift system were involved, providing comprehensive coverage of shift work in the United Kingdom. Matching was achieved for sex, age, workplace, and occupation. Absence records included certified sickness, short sickness, and non-medical absence.

The overall results showed that shift workers had less absence of all three types than their colleagues on day work, this difference being most marked in the numbers of men having several episodes. No significant differences were found in the diagnostic pattern of certified absence. Comparisons between day work and each of the six types of shift work did not provide any definite conclusions as to their relative merits as far as absence is concerned. The results from the different organizations were not wholly consistent, but a substantial majority followed the general trend.

     

Efficacy of intermittent exposure to bright light for treating maladaptation to night work on a counterclockwise shift work rotation

Objectives:Rotating shift work is associated with adverse outcomes due to circadian misalignment, sleep curtailment, work-family conflicts, and other factors. We tested a bright light countermeasure to enhance circadian adaptation on a counterclockwise rotation schedule.Methods:Twenty-nine adults (aged 20–40 years; 15 women) participated in a 4-week laboratory simulation with weekly counterclockwise transitions from day, to night, to evening, to day shifts. Each week consisted of five 8-hour workdays including psychomotor vigilance tests, two days off, designated 8-hour sleep episodes every day, and an assessment of circadian melatonin secretion. Participants were randomized to a treatment group (N=14), receiving intermittent bright light during work designed to facilitate circadian adaptation, or a control group (N=15) working in indoor light. Adaptation was measured by how much of the melatonin secretion episode overlapped with scheduled sleep timing.Results:On the last night shift, there was a greater overlap between melatonin secretion and scheduled sleep time in the treatment group [mean 4.90, standard deviation (SD) 2.8 hours] compared to the control group (2.62, SD 2.8 hours; P=0.002), with night shift adaptation strongly influenced by baseline melatonin timing (r²= -0.71, P=0.01). While the control group exhibited cognitive deficits on the last night shift, the treatment group’s cognitive deficits on the last night and evening shifts were minimized.Conclusions:In this laboratory setting, intermittent bright light during work hours enhanced adaptation to night work and subsequent readaptation to evening and day work. Light regimens scheduled to shift circadian timing should be tested in actual shift workers on counterclockwise schedules as a workplace intervention.     

Indexes of cardiac autonomic profile detected with short term Holter ECG in health care shift workers: a cross sectional study

Background:The assessment of early effect of shift work-related circadian rhythms desynchronization and work-related stress in health care workers has gained a primary role among the duties of the occupational physician.Objectives:Aim of our study was to assess the cardiac autonomic modulation through quantification of sinus rhythm variability, as an index of the adaptability to shift work of the cardiovascular system in healthcare shift workers.Methods:We measured Heart Rate Variability (HRV) by short-term (60 minutes) Holter Electrocardiography (ECG) during the regular duties in the respective department of 42 healthcare workers (31 nurses and 11 physicians) of an Italian Hospital (12 male and 30 females, aged 24-58 years), working on 3 shifts with a forward fast rotation with rest at the end of the night shift (7 am - 2 pm; 2 pm - 10 pm; 10 pm - 7 am) or in a fixed daytime shift (8 am - 2 pm). Measurements were all performed between 9 am and 12 am for fixed day workers and between 9 am and 12 pm or between 10 pm and 1 am for shift workers. The following HRV parameters were compared between the subgroups of shift workers and daytime workers: mean heart rate (HR), standard deviation of all normal RR (NN) intervals (SDNN), standard deviation of the averages of NN intervals in all 5-minute segments of a recording (SDaNN) and the triangular index (the integral of the density distribution divided by the maximum of the density distribution). We used parametric tests for independent series to compare HRV parameters by subgroups within the study subjects. We also tested correlation between the variables of interest and the association between HRV and shift work modality, along with other covariates, by means of a multiple linear regression analysis.Results:We found significantly lower values of SDaNN in shift workers compared with workers engaged solely on day shifts (50.80 ms vs 66.71 ms; p=0,014). The mean heart rate did not show any significant difference between day workers and shift workers (85.78 bmp vs 85.53 bpm respectively). Multivariate analysis showed a significant association between SDNN and female gender and age, while no significant associations were found between HRV and shift work.Discussion:The autonomic control of the heart rhythm could be disrupted by desynchronization of the biological rhythm secondary to the organization of shift work and night work. Shift work is an important factor of social and biological distress, influencing the adaptability of the cardiovascular system to stimuli and demands of work organization.Key words: Shift work, heart rate variability, health personnel     

Diurnal variations of blood pressure in shift workers during day and night shifts

Summary The dependence of blood pressure upon internal rhythms and the short-term effects of shift rota on the blood pressure were investigated in shift workers. Blood pressure was measured every 30 min using automatic recorders for 24 h in 17 physically working men in a chemical factory during their morning and night shifts. Mean 24-h blood pressures were identical in the morning and night shifts. There were no differences of the mean blood pressure between the respective sleeping phases or between the working periods. The amplitudes of circadian blood pressure variations were equal. There was a phase difference of 8 h corresponding to the lag between the working periods. At this 8-h lag the hourly means of the 24-h blood pressure were closely correlated (r = 0.69). Comparisons of 24-h blood pressure profiles during the first and last days of a night shift week showed that the effects of night work on the blood pressure were already fully developed within the first 24h (r = 0.86). Thus the diurnal variations of the blood pressure are determined by the working and sleeping periods and largely independent of endogenous rhythm. There is no short-term alteration of the mean 24-h blood pressure after shift rota.     

Absence from work in relation to length and distribution of shift hours

Walker, J., and de la Mare, Gwynneth (1971). Brit. J. industr. Med., 28, 36-44. Absence from work in relation to length and distribution of shift hours. A long period on night shift or even permanent night work has sometimes been suggested for those on continuous shift work to allow circadian rhythms to adapt. As the weekly hours of work have been reduced there is some evidence that a permanent night shift is practical, and about 12% of all shift workers are on this type of work. However, the case for permanent night shift must be established on grounds of both effectiveness and acceptability.

The present study compares the absence experience, including sickness absence, of permanent day workers and permanent night workers matched for age and job in three undertakings which contained a range of working conditions.

The question of the relationship between absence from work and total hours worked including overtime has been reopened, and in comparing absence from work according to the type of shift the total hours worked must also be taken into account. The relationship between the average hours when a man was at work and the amount of absence was tested. The men in the three undertakings worked a wide range of voluntary overtime.

The results showed that in two undertakings long-term absence, mainly sickness absence, was higher on the night shift than on the day shift; and, in the third, absence was about the same on the two shifts. As the work load was less in the two undertakings with a higher absence on the night shift it was suggested that selective factors were operating. These results may be contrasted to studies which have compared the absence of rotating shift workers and day workers.

In all three undertakings there was a tendency for absence to be less among high overtime workers than among those who worked medium or small amounts of overtime, although the trends were not consistent. There was no evidence at all that high overtime and absence from work were positively associated.

The implications of these results are discussed.

     

Shift work and subfecundity: a causal link or an artefact?

Methods: From 1 March 1998 to 1 May 2000, 39 913 pregnant women were enrolled in the DNBC. Data on job characteristics and TTP (0–2, 3–5, 6–12, and >12 months) were used for 17 531 daytime workers and 3907 shift workers who had planned the pregnancy. Fecundity odds ratios (ORs) were calculated with 95% confidence intervals using the discrete time survival analysis techniques performed by logistic regression. An OR above 1 expresses a shorter TTP and then a higher fecundity. Potential confounders, such as age at conception, gravidity, prepregnant body mass index, smoking, and alcohol consumption, as well as occupational characteristics, were also included in the model.

Results: Fixed evening workers and fixed night workers had a longer TTP. Compared with daytime workers, the adjusted ORs were 0.80 (95% CI 0.70 to 0.92) for fixed evening workers, 0.80 (95% CI 0.63 to 1.00) for fixed night workers, 0.99 (95% CI 0.91 to 1.07) for rotating shift (without night) workers, and 1.05 (95% CI 0.97 to 1.14) for rotating shift (with night) workers. When analysis was restricted to nulliparous women, the estimates remained unchanged. The proportions of unplanned pregnancies and contraceptive failures were higher among fixed evening and fixed night workers.

Conclusions: There was no unequivocal evidence of a causal association between shift work and subfecundity. The slightly reduced fecundity among fixed evening workers and fixed night workers may be mediated by pregnancy planning bias or differential options for sexual contacts.

     

Nutrition and 3‐shift work. The 24‐hour intake of energy and nutrients

Food consumption was studied (repeated 24‐hour recalls) during five days (four different work shifts and days off) in 16 healthy Swedish male papermill workers (rotating 3‐shift). A comparison (energy, fourteen nutrients) between 24‐hr periods showed a higher intake of energy and five nutrients during the 12 h work shift day compared to days off. The mean 24‐hr energy‐intake varied between 16,7 MJ (12 h work shift) and 13,3 MJ (days off). When only work hours were considered, the intake of energy and six nutrients were significantly higher during the morning‐shift compared to the night‐shift. There were no differences in the quality of the diet or the coffee consumption between 24‐hr days or between 8‐hr shifts. It was concluded that shift work affects 24 h nutrient intake to a very limited extent, although the distribution within 24 hours may vary.     

轮班作业女工排卵日前后的夜班次数对妊娠结局的影响

探讨轮班作业女工排卵日前、后的夜班次数对妊娠结局的影响。方法 以安徽某纺织厂符合研究条件的轮班作业女工作为研究队列进行问卷调查，了解一般信息以及某些职业因素的暴露情况。并收集每日晨尿直到临床怀孕或入队列满1年仍未怀孕，同时每日记录当日班次及留尿情况。所有尿样用免疫放射测定分析方法（IRMA）测定绒毛膜促性腺激素（hCG)来判断研究对象是否怀孕，并结合卵泡刺激素（FSH）、雌激素与孕激素的代谢产物雌酮结合物（E1C）、孕二醇－3－葡萄糖酸甙（PDG）等的测定来确定早期妊娠推选丢失。结果 以临床可见的自然流产组（12人）及经实验室确诊的早早孕丢失组（18人）为病例组，临床活产为对照组（44人）进行单因素和多因素Logistic回归分析，无论单因素还是多因素分析，在排卵日前、后3天的夜班次数和临床可见的自然流产有关（单因素：OR＝2.48,95%CI为1．10－5．60；多因素：OR＝3．90，95％CI为1．28－11．90）。与早早孕丢失组也有一定的关系，但差异无显著性（P＞0．05）。结论 在轮班作业女工中，排卵日附近的夜班作业可能与自然流产存在一定的关系。     

Work at night and breast cancer - report on evidence-based options for preventive actions

In 2007, the International Agency for Research on Cancer classified shift work involving circadian disruption as probably carcinogenic to humans (group 2A), primarily based on experimental and epidemiologic evidence for breast cancer. In order to examine options for evidence-based preventive actions, 16 researchers in basic, epidemiological and applied sciences convened at a workshop in Copenhagen 26-27 October 2011. This paper summarizes the evidence from epidemiological and experimental studies and presents possible recommendations for prevention of the effects of night work on breast cancer. Among those studies that quantified duration of shift work, there were statistically significant elevations in risk only after about 20 years working night shift. It is unclear from these studies whether or not there is a modest but real elevated risk for shorter durations. Hence, restriction of the total number of years working night shift could be one future preventive recommendation for shift workers. The diurnal secretion of melatonin by the pineal gland with peak in secretory activity during the night is a good biochemical marker of the circadian rhythm. Disruption of the diurnal melatonin secretion pattern can be diminished by restricting the number of consecutive night shifts. Reddish light and reduced light intensity during work at night could potentially help diminish the inhibitory activity of light with strong intensity on the melatonin secretion, but further mechanistic insight is needed before definite recommendations can be made. Earlier or more intensive mammography screening among female night shift worker is not recommended because the harm-benefit ratio in this age group may not be beneficial. Preventive effects of melatonin supplementation on breast cancer risk have not been clearly documented, but may be a promising avenue if a lack of side effects can be shown even after long-term ingestion. Women with previous or current breast cancer should be advised not to work night shifts because of strong experimental evidence demonstrating accelerated tumor growth by suppression of melatonin secretion. Work during the night is widespread worldwide. To provide additional evidence-based recommendations on prevention of diseases related to night shift work, large studies on the impact of various shift schedules and type of light on circadian rhythms need to be conducted in real work environments.     

Comparing performance on a simulated 12 hour shift rotation in young and older subjects

OBJECTIVES—To simulate a 12 hour shift rotation and measure the difference in performance if any, between older and younger subjects. Significant reductions in neurobehavioural performance during shift work and particularly night work have long been recognised. There are conflicting reports of the effects of 12 hour shifts on performance, alertness, and safety. Furthermore, research suggests that older shift workers have more sleep disruption and maladaption to shift work. When this is combined with longer hours at work there may be considerable reductions in performance for older compared with younger workers.
METHODS—Thirty two subjects were allocated to groups according to age. Group one had 16 subjects with a mean (SD) age of 21.2 (2.7) years, and group two had 16 subjects with a mean (SD) age of 43.9 (6.8) years. Subjects came to the laboratory for six consecutive days and completed a simulated 12 hour shift rotation consisting of two 12 hour day shifts (0700-1900), followed by two 12 hour night shifts (1900-0700). During the work period subjects completed a computer administered neurobehavioural performance task every hour.
RESULTS—Performance for the older subjects was consistently lower than for the younger subjects. There was a significant difference in performance across the shift between older and younger subjects. There was a significant change in performance across the shifts in the older subjects, such that performance significantly increased across the day shifts and decreased across the night shifts. By contrast, the younger subjects were able to maintain performance across both day shifts and the second night shift.
CONCLUSIONS—There are significant differences in performance of older and younger subjects during a simulated 12 hour shift rotation. Future studies both in the field and the laboratory would be useful in determining whether this is typical and if there are any important consequences for the older worker on 12 hour shifts.


Keywords: 12 hour shifts; performance; age; sleep reduction; circadian rhythms; night work     

Changes in psychophysiological functions during night shift in nurses

 The effects of changing from a full-day to a half-day shift work before a night duty shift on physiological and psychological functions during the night shift were investigated in 12 healthy unmarried nurses working on the same ward of a university hospital. Three shift patterns, i.e., a day shift following a day shift, a night shift following a day shift, and a night shift following a half-day shift, were studied in terms of physical activity level, sympathetic and parasympathetic activity levels, cortisol, prolactin, NK cell activity, and changes in mood states. The change to the half-day shift increased the duration of sleep before night duty by about 86 min and brought wake-up times forward by about 1 h, resulting in increases in rest and time before work. In addition, the change was revealed to reduce the influence of reversed-phase circadian rhythms on autonomic nervous activity during the night shift. The score for sleepiness was significantly lower at 0500 hours following a half-day shift. There were some marginal but not significant differences in the scores reflecting the degree of vigor, tiredness and irritation during the night shift. Although the prolactin concentration was significantly decreased at the start of the night shift after the half-day shift, there was no difference in cortisol concentration or NK cell activity between the usual night shift after a day shift and the night shift after the half-day shift. The half-day shift was not observed to cause any marked change in the fixed biorhythms of these nurses. The cortisol and NK cell activity levels were low during the night shift, suggesting that the night shift itself is a high stress level, which is prejudicial to biodefense. Received: 1 August 1995/Accepted: 8 March 1996