The impact of a week of simulated night work on sleep, circadian phase, and performance

Methods: Fifteen healthy subjects participated in an adaptation and baseline night sleep, directly followed by seven simulated eight-hour night shifts (2300 to 0700 hours). At the end of each shift they were taken outside and exposed to natural light for 20 minutes. They then slept from approximately 0800 hours until they naturally awoke.

Results: There was a significant increase in mean performance on a visual psychomotor vigilance task across the week. Daytime sleep quality and quantity were not negatively affected. Total sleep time (TST) for each of the daytime sleeps was reduced, resulting in an average cumulative sleep debt of 3.53 hours prior to the final night shift. TST for each of the daytime sleep periods did not significantly differ from the baseline night, nor did TST significantly vary across the week. There was a significant decrease in wake time after sleep onset and sleep onset latency across the week; sleep efficiency showed a trend towards greater efficiency across the consecutive daytime sleeps. Hours of wakefulness prior to each simulated night shift significantly varied across the week. The melatonin profile significantly shifted across the week.

Conclusions: Results suggest that under optimal conditions, the sleep debt that accumulates during consecutive night shifts is relatively small and does not exacerbate decrements in night-time performance resulting from other factors. When sleep loss is minimised, adaptation of performance during consecutive night shifts can occur in conjunction with circadian adaptation.

     

Effects of 2-hour nighttime nap on melatonin concentration and alertness during 12-hour simulated night work

A nighttime nap is expected to mitigate melatonin suppression during night work by blocking light input to the retina, but it is unclear. In the present study, we investigated the effects of a nap break on melatonin level, subjective sleepiness, and vigilance performance during simulated night work. Eleven healthy young males (mean ± SD age: 22.2 ± 4.1 years) participated in counterbalanced crossover design experiments with two conditions (nap vs. no nap). The subjects performed 12-hour simulated night work from 21:00 to 09:00 h (illuminance: ∼500 lx). Subjects with a nap condition took a nap for 2 hours in a dark room from 03:00, while subjects with a no nap condition continued the simulated night work. The results showed that immediately after the 2-h nap break, the melatonin level at 05:00 h temporarily recovered from light-induced melatonin suppression during the simulated night work but significantly suppressed again at 07:00 and 09:00 h. Subjective alertness and vigilance performance were impaired immediately after the nap break but subsequently enhanced. The results suggest that a single nap break for 2 hours could be a strategy to enhance alertness during the last part of night shift but inadequate for mitigating melatonin suppression.     

Changes in neuropsychological symptoms and moods among tanker drivers exposed to gasoline during a work week

The purpose of this study was to investigate changes in theneuropsychological symptoms and moods among tanker drivers duringa work week and the associations of the changes with exposureto gasoline (including 10% methyl-tert-butyl ether). The targetgroup for the study consisted of 101 road tanker drivers fromthree Finnish oil companies in various parts of Finland. Thecontrol group consisted of 100 milk delivery drivers from twomilk companies from the same localities. Interviews were conductedbefore the work week and at the end of the same work week. Standardizedsymptom questionnaires were used to elicit responses to questionsabout symptoms and moods. In the questionnaires tanker driversscored significantly higher in the fatigue scale at the endof the work week than before the work week. Our findings showthat tanker drivers with long exposure to gasoline during thework week developed significantly higher changes in fatiguescores than drivers with short exposure. During the exposuresituations, 20% of tanker drivers reported acute symptoms (headache,dizziness, nausea, dyspnoea, irritation of saliva excretion)at the end of work week voluntarily. These symptoms have beenconnected, with MTBE (methyl-tert-butyl ether) exposure, amongothers. Exposure to MTBE during the work week can be reasonfor acute symptoms among the tanker drivers in this study.     

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.     

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     

Alcohol consumption and urinary concentration of 6-sulfatoxymelatonin in healthy women

Consumption of alcoholic beverages may suppress circulating melatonin levels at night, possibly resulting in an increase in circulating estrogen. An increased estrogen burden could increase the risk of breast cancer. This study was designed to investigate whether alcohol consumption is associated with a decrease in nighttime melatonin levels in a group of healthy women. A total of 203 randomly selected healthy women between the ages of 20 and 74 years were recruited for a broader study of the effects of exposure to power-frequency magnetic fields on nocturnal levels of urinary 6-sulfatoxymelatonin. For the purposes of this analysis, data collection consisted of the following during two seasons of the year: (1) an in-person interview, (2) a daily activity diary, and (3) nocturnal urine collection for each of 3 consecutive nights. We found that the nocturnal urinary concentration of the primary metabolite of melatonin (6-sulfatoxymelatonin) decreased in a dose-dependent manner with increasing consumption of alcoholic beverages in the preceding 24-hour period, after taking into account the independent effects on melatonin of age, hours of darkness, use of medications that affect melatonin levels, and body mass index. A categorical analysis revealed no effect of one drink, but a 9% reduction with two drinks, a 15% reduction with three drinks, and a 17% reduction with four or more drinks. It remains unknown whether such a change could affect estrogen levels or breast cancer risk.     

The impact of a meal,snack, or not eating during the night shift on simulated driving performance post-shift

Objective:The commute home following a night shift is associated with an increased risk for accidents. This study investigated the relationship between food intake during the night shift and simulated driving performance post-shift.Methods:Healthy non-shift working males (N=23) and females (N=16), aged 18–39 years (mean 24.5, standard deviation 5.0, years) participated in a seven-day laboratory study and underwent four simulated night shifts Participants were randomly allocated to one of three conditions: meal at night (N=12; 7 males), snack at night (N=13; 7 males) or no eating at night (N=14; 9 males). During the night shift at 00:30 hours, participants either ate a large meal (meal at night condition), a snack (snack at night condition), or did not eat during the night shift (no eating at night condition). During the second simulated night shift, participants performed a 40-minute York driving simulation at 20:00, 22:30, 01:30, 04:00, and 07:30 hours (similar time to a commute from work).Results:The effects of eating condition, drive time, and time-on-task, on driving performance were examined using mixed model analyses. Significant condition×time interactions were found, where at 07:30 hours, those in the meal at night condition displayed significant increases in time spent outside of the safe zone (percentage of time spent outside 10 km/hour of the speed limit and 0.8 meters of the lane center; P<0.05), and greater lane and speed variability (both P<0.01) compared to the snack and no eating conditions. There were no differences between the snack and no eating conditions.Conclusion:Driver safety during the simulated commute home is greater following the night shift if a snack, rather than a meal, is consumed during the shift.