Imagine life on a spaceship headed to Mars. You and your five crewmates work, exercise, and eat together every day under the glow of fluorescent lights. As the months pass, the sun gets dimmer and communication with Earth gets slower. What does this do to your body? According to an Earth-based experiment in which six volunteers stayed in a windowless "spaceship" for nearly a year and a half, the monotony, tight living space, and lack of natural light will probably make you sleep more and work less. Space, for all intents and purposes, turns you into a couch potato.

A mission to Mars would take about 520 days-including the months it takes to travel there and back and the time spent on the Red Planet. But the longest any human has spent off Earth is 437 days straight, a record set in 1995 by Russian cosmonaut Valeri Polyakov, who was orbiting relatively close to home in the space station Mir. Polyakov seemed to bear the burden of long space flight fairly well, but many questions remain about just what a long mission farther from the sun would do to astronauts' ability to sleep and keep energized.

For answers, scientists turned to the Mars500 mission simulation in Moscow, which ended in November 2011. Six crewmen-who are an international mix of astronaut trainers, engineers, and doctors between 27 and 38 years old-stayed in a series of tunnellike chambers and played out the 520-day mission to Mars. "It was thought to be a safe way to try to begin to understand those questions instead of just launching into an extremely long space flight," says study co-leader and sleep scientist David Dinges of the University of Pennsylvania. The crew could communicate with the outside world, but with a time delay depending on how far the mission was supposed to be from Earth. Halfway through the mission, the volunteers even worked on a simulated martian landscape inside a large room with a sandy floor and a black ceiling dotted with fake stars. Throughout the experiment, video cameras and activity-monitoring wristwatches recorded the crew's movements, enabling scientists to observe how often the crew members slept and how much of their time awake was spent lazing about.

The study's finding, reported online Monday in the Proceedings of the National Academy of Sciences, was that the crew suffered from hypokinesis, meaning "they simply moved less," Dinges says. The volunteers moved less while awake and asleep, and spent more waking hours each day engaged in restful activities-playing video games, reading books, or watching movies. The crewmembers' wristwatches, which were equipped with light sensors, showed that the more lethargic they became, the more they shunned the lighted parts of the ship. By the final few months of the mission, three of the crewmembers slept about an hour more per day than they had at the beginning of the simulation.

The beds were small, recalls French crew member and flight engineer Romain Charles, and so narrow that he often had to sleep with his arm across his face. "I just had to learn to sleep like that," he says. But even though he adapted to the sleeping arrangements, it became difficult to take on intellectually laborious tasks, like improving his Russian language skills, in the final few months of the mission. Instead, he spent his spare time playing the video game Counter-Strike. "It really helped me get through that," Charles remembers.

It wasn't until the final 20 days of the mission that the crew, excited for their seclusion to finally end, became nearly as energetic as they were when the mission began.

These findings are important because a crew in space needs to be at peak performance to work competently and act quickly should an emergency strike, says psychologist Gloria Leon of the University of Minnesota, Twin Cities, who was not involved with the study. "What they're showing is that NASA and other organizations need to pay close attention to developing some measures that will prevent hypokinesis."

The unnatural conditions of future space journeys have to better emulate the natural qualities of life on Earth, Dinges says. For example, the fluorescent lighting in the Mars500 chamber mostly emitted wavelengths in the green and yellow part of the visible light spectrum, but not much blue. Blue light is important, Dinges notes, because the light from the Earth's natural dawn is largely blue and signals the brain that it's time to wake up. The crew would also have to hold a tight schedule for meals and physical activity.

The sleep times of the Mars500 crew were not quite synchronized, which would increase the risk of accidents. "Imagine a watch that's running 24 hours and 20 seconds a day and another that's got a 24-hour day exactly," says Dinges. "Over time, those two watches will go completely out of sync, and that's what we want to make sure doesn't happen to a crew." One crewman seemed chronically sleep deprived, the researchers noted, sleeping less as the simulation progressed and reporting poor-quality sleep throughout the whole mission. Another crew member's sleeping habits went completely off schedule. He lived on a 25-hour sleep-wake cycle and frequently snoozed while the rest of the crew was awake.

The findings show that agencies recruiting astronauts for such a long mission should take their candidates' established sleep habits into account, says physiologist Derk-Jan Dijk of the University of Surrey in the United Kingdom, who was not involved with the study. "You definitely want to be sure that these people, while going about their normal lives, are not already extreme late sleepers, for example."

- - -

This is adapted from ScienceNOW, the online daily news service of the journal Science. http://news.sciencemag.org