Getting sleep will be one of the biggest challenges facing astronauts in any future manned mission to Mars, according to a study into six men who spent 520 days and nights in a confined "spacecraft" during a simulated trip to the planet.
The Mars500 project began on 3 June 2009 when three Russians, an Italian, a Frenchman and a Chinese man entered a sealed experimental facility in Moscow without access to natural light, fresh air or direct face-to-face contact with any other human being.
For the next 18 months they carried out a battery of pre-arranged daily duties to simulate a return mission to Mars, with a 30-day interlude in the middle of the programme where they were allowed to explore another enclosed area designed to simulate the surface of the red planet.
A study, published in the Proceedings of the National Academy of Sciences, into how each man coped with the psychological and physical constraints of the mission – carried out entirely under artificial light – has found that there were wide differences in their wake-sleep patterns.
One man's circadian rhythm shifted from a 24-hour period to a 25-hour period, which meant that on every 12th day his body was telling him that it was midnight at the same time that it was midday for everyone else.
And while most of the crew began to sleep for longer periods as the mission progressed and boredom set in, one individual became chronically sleep deprived.