The continuous daylight conditions of summer in Antarctica are known to interfere with physiological functions such as sleep patterns and the release of melatonin, a hormone associated with circadian rhythms and sleep. Now, a study offers new information about why people in this region sleep poorly, and suggests that social behavior may also play a role. The study, published ahead of print in the Journal of Applied Physiology, was chosen as an APSselect article for March.
Antarctica, located at the tip of the Southern Hemisphere, experiences 24-hour daylight and no darkness for several months of the year (Antarctic summer). An expedition team stationed in Antarctica during the summer followed their daily work routine and volunteered for sleep studies at night. Belgian researchers monitored the volunteers' sleep stages and recorded the secretion of cortisol (a stress hormone also associated with wakefulness) and melatonin.
In normal circumstances, we begin the night with proportionally more deep sleep that provides the body with physical recovery. As morning approaches, sleep patterns contain more dream sleep. The research team found the reverse was true in the Antarctic expedition workers—their dream sleep occurred earlier, with a deep sleep period at the end of the night. In addition, melatonin secretion—a process that helps you fall asleep—was delayed by several hours. Melatonin is sensitive to light and is usually released in the darkness of night. Members of the expedition team also reported "morning sleepiness, which waned with exposure to sunlight." This may be due to the delayed release of melatonin, which remained high in the body upon waking, explained Nathalie Pattyn, first author of the study.
To the researchers' surprise, cortisol secretion remained normal, with the highest levels secreted in the morning. Typically, melatonin and cortisol have an inverse relationship: When one hormone is high, the other is low. The common schedule of the expedition members—who worked and slept at the same time—kept the cortisol from delaying its release. If the expedition team had been on different schedules, the timing of cortisol secretion would have likely changed. These findings warrant more study about how social behavior modifies physiological function explained Pattyn, because it "shows the story behind sleep complaints in Antarctica is more complicated" than just being exposed to continuous sunlight.