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The isolation of melatonin was first reported in 1958. Since the demonstration that pineal melatonin synthesis reflects both daily and seasonal time, melatonin has become a key element of chronobiology research. In mammals, pineal melatonin is essential for transducing day-length information into seasonal physiological responses. Due to its lipophilic nature, melatonin is able to cross the placenta and is believed to regulate multiple aspects of perinatal physiology. The endogenous daily melatonin rhythm is also likely to play a role in the maintenance of synchrony between circadian clocks throughout the adult body. Pharmacological doses of melatonin are effective in resetting circadian rhythms if taken at an appropriate time of day, and can acutely regulate factors such as body temperature and alertness, especially when taken during the day. Despite the extensive literature on melatonin physiology, some key questions remain unanswered. In particular, the amplitude of melatonin rhythms has been recently associated with diseases such as type 2 diabetes mellitus but understanding of the physiological significance of melatonin rhythm amplitude remains poorly understood.
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Epidemiological studies have shown that shift workers are at a greater risk of developing cardiovascular disease which may, in part, be related to metabolic and hormonal changes. Partial sleep deprivation, a common consequence of rotating shift work, has been shown to affect glucose tolerance and insulin sensitivity. The current study investigated the effects of one night of total sleep deprivation, as a proxy for the first night shift, on postprandial glucose, insulin and lipid (triacylglycerols (TAGs) and non-esterified fatty acids (NEFAs)) responses under controlled laboratory conditions in shift workers and non-shift workers. Eleven experienced shift workers (35.7±7.2 years, mean±s.d.) who had worked in shifts for 8.7±5.25 years were matched with 13 non-shift workers who had worked for 32.8±6.4 years. After an adaptation night and a baseline sleep night, volunteers were kept awake for 30.5 h, followed by a nap (4 h) and recovery sleep. Blood samples were taken prior to and after a standard breakfast following baseline sleep, total sleep deprivation and recovery sleep. Basal TAG levels prior to the standard breakfast were significantly lower after sleep deprivation, indicating higher energy expenditure. Basal NEFA levels were significantly lower after recovery sleep. Postprandial insulin and TAG responses were significantly increased, and the NEFA response was decreased after recovery sleep, suggestive of insulin insensitivity. Although there were no overall significant differences between non-shift workers and shift workers, non-shift workers showed significantly higher basal insulin levels, lower basal NEFA levels, and an increased postprandial insulin and a decreased NEFA response after recovery sleep. In future, the reasons for these inter-group differences are to be investigated.
