Without daily sunsets and sunrises, astronauts often have a difficult time sleeping. CNN reports:

Dinges co-authored research on a spaceflight simulation experiment known as Mars500, developed by the Russian Academy of Sciences. For most of 2010 and 2011, six men were kept in isolation for 520 days, tending to daily tasks, simulating 20-minute communication delays with Earth and even walking on a fake Mars. Meanwhile, their sleep, food intake and activity were monitored constantly.
The men had vastly different responses to the 17-month experiment. Only two of the crewmembers seemed to cope well and sleep regularly, Dinges said. Among the others, one suffered increasing insomnia, and one became more lethargic and slept more than the others. Yet another became depressed, and the remaining crewmember settled into 25-hour sleep cycle, staying awake when his crewmembers were asleep.
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