The crew of the Mars500 experiment relax during the 'return trip' of the analog mission to Mars.ESA/Mars500

Most of the crewmembers participating in a 520-day isolation study to simulate a Mars mission suffered from insomnia and other sleep disorders, a condition that left them impaired during their waking hours as well.

The finding indicates that future long-duration space travelers will need specialized lighting to replicate Earth's day-night cycles as well as other countermeasures to maintain healthy circadian rhythms, say researchers who conducted one of more than 90 investigations in the joint Russian-European Mars500 project.

"The assumption has been with the six-month space station missions that anybody can tolerate them. OK, you have trouble with your sleep or something but you're only up there six months, it won't last. If you go on an exploration mission, you'll adapt. But our study shows that's not true," sleep researcher David Dinges, with the University of Pennsylvania's Perelman School of Medicine, told Discovery News.

"The people did not adapt. In fact, these problems just cumulatively created a greater and greater physiological and behavioral burden on the crewmember," he said.

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Dinges' subjects were six men -- three Russians, two European and one Chinese -- who spent 520 days sealed inside a spaceship-like, windowless chamber at the Russian Institute for Biomedical Problems in Moscow for a Mars mission simulation.

"We wanted to get some idea of what will happen when a motivated, high-performing crew is confined in a spacecraft-like environment for a full 17 months, simulating a mission to Mars and back," Dinges said.

So far, only four people -- all Russian -- have made spaceflights lasting more than a year. The single longest human spaceflight was a 437-day mission aboard the Russian Mir space by Valery Polyakov, a physician, in 1994 and 1995.

The 520-day ground simulation, which ended on Nov. 4, 2011, showed that four of the six crewmembers suffered from sleep disorders, including one subject who ended up on a 25-hour day and another who split his sleep into two cycles within a 24-hour period. Such disconnects could affect a crew's ability to work as a team, among other problems, researchers write in a study published last week in the Proceedings of the National Academy of Sciences.

"The differences suggest that some people are more vulnerable than others to the loss of light cues for biological timing and on the effects of confinement on their ability to sleep, or to sleep well. We don't have an obvious predictor for that," Dinges said.

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Being susceptible to circadian disconnects wouldn't necessarily eliminate someone from consideration for a Mars missions, he added.

"We'd just make sure they had countermeasures to make sure they maintain their sleep/wake cycle better," Dinges said.

Further research is needed to assess the impacts of diet and exercise on the crew's sleep patterns and attentiveness while awake.

"The timing of exercise should not be arbitrary, as it is now, but should be structured to reinforce circadian biology. The same with food. We believe the timing of food and its relationship to bowel and digestion is critical" to maintaining circadian rhythm, Dinges said.

NASA and Russia plan to begin year-long missions on the International Space Station in part to learn more about how extended stays in space impact the human body and performance.

"In the morning you wake up, you're at work. When you go to sleep, you're also at work," said astronaut Scott Kelly, who is slated to begin NASA's first year-long spaceflight in 2015.

"Imagine being in your office for a whole year and you never get to leave. That is a challenge, it presents its own set of issues, but I think I'm up for it and I look forward to it," Kelly told reporters at press conference last month.

Mars500 participants wore wrist monitors that relayed information every minute about their movements. Other data came from video feeds and self-assessment tests.