CORTISOL AND MELATONIN RHYTHMS DISSOCIATION DURING AN ANTARCTIC SUMMER EXPEDITION: EVIDENCE FOR TWO DISTINCT CIRCADIAN OSCILLATORS

Introduction and Objectives: Sleep complaints are consistently cited as the most prominent problem in Arctic and Antarctic expeditions. Continuous bright light exposure in the summer, and continuous darkness in the winter suggest a fundamental disturbance of circadian sleep-wake regulation in this environment. However, there is no clear evidence to date of a consistent circadian disruption, nor of an established relationship to the sleep complaints. Sleep-wake regulation and circadian rhythmicity of cortisol and melatonin were investigated during a 4 month summer expedition in Antarctica. Materials and Methods: After an habituation night and acclimatization to the environment, polysomnography was performed in 21 healthy male subjects, free of medication. Circadian rhythms were determined with a 18 hours profile (saliva sampling every 2 hr) of cortisol and melatonin. All data collection was performed during the continuous illumination of the Antarctic summer. Results: Polysomnography results showed, in addition to high sleep fragmentation, a dramatic decrease in slow wave sleep (SWS) and an increase in REM sleep. Furthermore, SWS occurred at the end of the night, rather than the beginning. Autonomic activation showed a concurrent variation, with a high proportion of low frequency heart rate variability, and a delayed occurrence of the high frequency component. Cortisol rhythmicity and serum levels were preserved, and secretion profiles were remarkably synchronized among participants. Melatonin secretion however, showed a severe phase delay, with no secretion onset as late as 24.00 and peak values around 06.00. Conclusion: The present results showa complete dissociation of cortisol and melatonin secretion profiles. The delayed and decreased SWS could be subtended by the phase delay in melatonin secretion. The modified autonomic regulation is related to the disturbed pattern of sleep stages. These findings suggest two distinct oscillators regulating cortisol, being more sensitive to social schedule, and melatonin, being more sensitive to photoperiod.