Research published in the Proceedings of the National Academy of Sciences suggests differences in the sleep patterns of the crew caused problems.
The findings suggest that not all current astronauts will be suited to interplanetary travel.
The Mars500 project investigated how crews would cope on a real mission.
Prof Mathias Basner, of the University of Pennsylvania – who was involved in the sleep study – says that the findings show that astronauts for any future Mars missions should be tested for their ability to cope without a natural day/night cycle.
“This illustrates that there are huge differences between individuals and what we need to do is select the right crew, people with the right stuff, and train them properly and once they are on the real mission to Mars,” he told BBC News.
Currently, no astronaut is in space for longer than six months on the International Space Station (ISS). The aim of the 17 month-long Mars500 project was to study the physical and psychological effects that the much longer journey to Mars might have on future astronauts.
Thesimulationinvolved six crew members: three Russians, two Europeans and one Chinese volunteer.
For much of the time, the men had only limited contact with the outside world. Their spaceship had no windows, and the protocols demanded their communications endured a similar time lag to that encountered by real messages as they travel the vast distance between Earth and Mars.
Nearly 100 different experiments were carried out to assess the impact of the journey on the men, and it is only now that the first results are emerging.
The sleep experiment is among the first to show what each crew member went through during their simulated mission.
The researchers found that one crew member lost his natural day/night rhythm completely. Instead of a 24-hour cycle, he slipped into a 25-hour day so after 12 days he was completely out of sync with his fellow crew mates. It was the middle of the night for him while his colleagues were working on the mission.
“You can imagine that that would be good during a real Mars mission when there are mission critical tasks planned during the day,” said Prof Basner.
“He became somewhat isolated. For 20% of the time this crew member was either the only crew member awake or the only person sleeping which could potentiallybeaproblemforteamcohesion,”he