Houston, we have a sleep disorder. That's essentially the greatest issue that faced astronauts who volunteered for a 17-month space simulation called the Mars500 project. It's not exploding oxygen tanks or a malfunctioning command module or a broken space heater or, shit, HAL. The failures were physiological, not mechanical.
Mars500 brought together three Russians, two Europeans and one Chinese crew member--all men--who volunteered to test the limits of the mind in the ultimate sensory deprivation chamber, a replica spaceship without windows, nestled in Moscow. Researchers studying the crew ran nearly 100 experiments on the men to gauge the impacts of protracted space travel. They even simulated the days-long lag in radio communications between Earth and Mars. Results of the sleep study are the first to be released since the project wrapped, and some of them read like science-fiction horror stories.
One crew member, for example, lost his circadian rhythm within days. Having evolved on Earth, humans are intrinsically in tune with both the revolutions of the planet around the Sun and the natural cycle of day and night. In space, that light-dark cycle disappears. The crew member wound up falling into a 25-hour day, which after a few days had dragged his rhythm far out of sync with those of his space buddies, making collaboration on basic tasks impossible. Their day was his night.