Mars500 crew. Source
This picture here is of the Mars500 crew. These training astronauts were put in a 17 month long test to see how they would cope with extended space travel, living cut off from their fellow man. The study revealed that sleep could be a major problem for a crew like this. Most of the men tested here began to sleep more and become less active while one of them suffered from chronic sleep deprivation. Another crew member also had an inverted sleep cycle to the rest of his comrades. These could all be problems for future astronauts who have to work together and be constantly alert in a hostile environment. One of the biggest factors leading to the sleep problem is thought to be the lighting. Since no natural light gets in, a person's day/night cycle is easily thrown off balance.
I recommend reading more about this here.