Abstract
There are many problems associated with night shift work, involving both the disruption of social activities and the desynchrony between internal biological clock timing and the forced regimen. Both short term problems (e.g. sleep deficiency, reduced alertness and reduced performance) and potential long term problems (e.g. coronary heart disease and diabetes) may be critically dependent on whether or not the worker is able to adapt to the shift regimen. In the first of two baseline studies, the 6-sulphatoxymelatonin rhythms of offshore oil workers on a two-week 12-hour night shift (1800h to 0600h) were shown to adapt to the regimen within the first seven days of the shift. The rates of phase shift (mean +/- sem) were 1.51 +/- 0.16 h/day (n=5), 1.32 +/- 0.41 h/day (n=5) and 1.77 +/- 0.31 h/day (n=17) for a winter drill crew, winter maintenance crew and summer maintenance crew respectively. The rate of adaptation was not significantly affected by the type of work conducted or the season. The second baseline study assessed the 6-sulphatox-5-onelatonin rhythms of offshore drill crews on a one-week day shift (1200h - 0000h), one-week night shift (0000h - 1200h) 'swing' shift. A crew studied in winter showed no change in their 6-sulphatoxymelatonin rhythm during night shift, while a crew studied in spring showed a significant phase advance to an acrophase position of 0051h +/-1.7 hours (mean +/- sem). This data, together with that of the first baseline study, indicate that both the type of shift and the season influence the direction and degree of adaptation. A simulation study was conducted to assess the hormonal and metabolic response to a test meal during the first night of night shift (1800h to 0600h). Both plasma glucose and insulin levels were elevated on night shift compared to day shift, suggesting a degree of glucose intolerance during this period. Treatment with bright light (1500 lux) throughout the night shift reduced the glucose intolerance observed and also lowered plasma triacylglycerol levels. The use of exogenous melatonin to help shift workers acclimatise to day shift following night shift was examined. Exogenous melatonin significantly increased sleep duration compared to placebo. There was also evidence of increased daytime napping and stabilization of sleep onset time under this treatment. While no specific adaptation rates could be observed, exogenous melatonin did not appear to have an adverse effect on the adaptation of the melatonin and alertness rhythms when taken at the desired bedtime. The data clearly show that adaptation of the melatonin rhythm to night shift is possible in certain environments. Further evidence of the deleterious effects of night shift on metabolic and hormonal responses to a nighttime meal has been observed, but the use of bright light treatment may be a potential solution to this problem. While the benefits of exogenous melatonin treatment on sleep may be of use offshore, an increased incidence of daytime napping, observed during melatonin treatment, may be of concern for its use in an offshore environment.