Mars travelers may want to pack some extra jammies, a study suggests, finding sleepiness and insomnia dogging astronauts in a space travel simulation of a 520-day trip to the Red Planet.

The sleep study results released Monday look at the six volunteers on the Mars 500 project, concluded in November. The cooperative effort by the Russian Institute for Biomedical Problems and the European Space Agency "sent" six young men on a simulated round-trip to Mars conducted within trailer-sized isolation chambers.

Aside from a brief visit to a simulated Mars landing site, they remained inside the chambers for the entire time, performing jobs similar to real ones needed on a real trip to Mars.

"A Mars mission trip will involve much longer periods of time in a spacecraft than anyone has experienced," says study author David Dinges of the University of Pennsylvania in Philadelphia. Only four people have lived more than a year in space. The record, 437 days on Russia's Mir space station, was set by cosmonaut Valery Polyakov.

A trip to Mars will probably take longer and involve communication cutoffs and delays, as well as radiation threats, even more trying than life aboard the International Space Station, Dinges says. "Sleep is a basic biological function and a concern for astronauts already, so given how it affects us, there is some concern about how well people will sleep on a Mars mission," Dinges says.

In the Proceedings of the National Academy of Science study, an international research team continuously tracked the movement, sleep and exposure to light of the Mars 500 volunteers during the entire simulation. They were tested weekly on how well they performed brain puzzles. "We were surprised to find a lot of torpor as the mission went on," Dinges says. "They moved less, and they slept more."

The crewmember who exercised the most during the simulated space trip slept the best, Dinges says.

The results show the importance of setting sleep cues tied to the body's internal "circadian rhythms" clock, says sleep expert Charles Szeizler of Harvard's Brigham and Women's Hospital in Boston, who was not part of the study.

He notes that the simulation astronauts approached eight hours of average sleep, about what most people need, only late in the mission, about 400 days in. "What they are calling torpor is looking more like normal sleep patterns after a period of overwork at the beginning of the mission combined with a sleep debt."

Both Dinges and Szeizler agree on the study pointing to the need to help future space travelers set their sleep schedules using appropriately bright lights to cue the body to nighttime and daytime sleep patterns. One of the crewmembers suffered runaway sleep patterns, sleeping during the "day" on the mission, dealing with insomnia and failing to improve on the mental assessment tests like crewmembers who received more sleep.

Mars 500 only went so far in answering the real questions about Mars travel, says astronaut health expert Nick Kanas of the University of California-San Francisco, "since real danger and microgravity were absent in this simulation, and since only one crew was studied."

U.S. astronaut Scott Kelly and Russian cosmonaut Mikhail Kornienko will spend a solid year aboard the International Space Station, twice as long as the usual stint, in 2015 as part of related efforts to prepare astronauts for long-distance space travel that space agencies hope to undertake in the next decade.

Dan Vergano, USA TODAY@dvergano