If humans ever journey to Mars, they will face an array of challenges: assault by cosmic rays, the erosion of bone mass and more subtle problems that could disrupt a mission’s success. Now experiments from an audacious, 17-month-long simulation of a trip to Mars and back show that the ability to concentrate and work together may decay unless preventive steps are taken to maintain sleep quality.

Six men agreed to hole up in a pressurized, spaceship-like environment in Moscow from June 2010 to November 2011 to help scientists lay the groundwork for an interplanetary mission. During that time, the pseudo-astronauts — three selected by the Russian Federation, two from the European Space Agency and one from the China National Space Administration — communicated with faux mission controllers (including delays to reflect the time it would take for radio transmissions to travel millions of miles) and performed mission-like activities. They even exited onto rocky, Mars-like terrain clad in heavy spacesuits to perform drills before simulating a return to Earth.

Scores of experiments were conducted during the mission. For the sleep study, a team led by scientists at the University of Pennsylvania and Baylor College of Medicine in Houston measured when crew members were active or at rest using devices called actigraphs. Worn on the wrist, the gadgets contained accelerometers that gauged crew members’ movement intensity at one-minute intervals. The devices also recorded the strength of light the wearer was exposed to at any given time.

The scientists also conducted attention assessments in which participants pressed the space bar of a computer keyboard every time a red dot appeared on the screen. Alertness was scored based on the response time and the number of times the space bar was pressed when no dot was present.

The data were loaded onto memory cards that were periodically ejected from the fake spacecraft along with the trash.

Three to four weeks after the spacecraft hatch was closed, the astronauts as a group became less and less active, the scientists reported Monday in Proceedings of the National Academy of Sciences. “The crew members increased their sleep time and even while awake, decreased their activity levels,” said Dr. Mathias Basner, a sleep researcher at Penn who worked on the study.

However, sleep quality varied strikingly among the six men, the scientists found. One man, isolated from the normal cues of light and dark experienced on Earth, abandoned the 24-hour day and adopted a 25-hour version, slowly moving in and out of sync with the sleep-wake cycles of his crewmates.

“This could potentially be dangerous during mission-critical tests,” Basner said.

Other crew members retained their 24-hour cycles, but one started sleeping at odd times of the day as well as at night. Another man slept less and less as the mission continued, unlike his peers. In total, four of the six crew members experienced some kind of sleep problem.

Performance on attention tests varied too, and one crew member performed poorly on the tests and said that he was struggling with them.

Toward the mission’s end, activity levels climbed even though the Russian controllers said they were not increasing the crew’s workload — probably because excitement was rising as the astronauts anticipated the end of their 520-day expedition, the authors said.

Though the mission was an overall success — nobody dropped out of the simulation and all survived to its end without incident — the variety of sleep disturbances among this small group was striking, said Josephine Arendt, a chronobiologist at the University of Surrey in England who studies circadian rhythm disturbances among people living through dark Antarctic winters. (Such over-winterers have long been studied as the best Earth-based analog to space travelers.)

Some of the sleep problems observed on the fake Mars mission could likely be traced to the absence of light at the right time of day or with the right intensity or spectrum, Arendt said. Most of the time, the crew members were in low or moderate light, the authors found, and the light was depleted in the blue spectrum that scientists know is important for keeping the human body clock on a 24-hour day.