A study has found that getting a good night's sleep will be one of the biggest challenges facing the first astronauts travelling to Mars.
Poor sleep was a key problem encountered by six volunteers during a 17-month simulated round trip to the Red Planet.
Throughout the mission, which included a month on the surface of Mars, the crew were confined in 550 cubic metre "spacecraft" and cut off from the outside world.
Scientists operating the facility in Russia monitored their medical and psychological condition, as well as activity levels and sleep patterns.
As the mission progressed, crew members became more sedentary and most experienced sleep disturbances, the data revealed.
Without normal sleep-wake activity cycles, their "body clock", or circadian rhythm, patterns became disrupted.
Sleep expert and member of the research team Dr Mathias Basner said: "Taken together, these measurements point to the need to identify markers of differential vulnerability to abnormal decrease in muscular movement and sleep-wake changes in crew members during the prolonged isolation of exploration spaceflight and the need to ensure maintenance of the Earth's natural circadian rhythm, sleep quantity and quality, and optimal activity levels during exploration missions."
Successful adaptation to long space missions will require environments that artificially mimic night and day cycles on Earth, said the scientists.
This will mean paying attention to timed light exposure, food intake and exercise.
The findings, published online by the journal Proceedings of the National Academy of Sciences, also have implications for Earth-bound living, according to the researchers.
They pointed out that many people in industrialised societies get little exercise, are exposed to artificial light for long periods and suffer sleep disruption due to their busy lifestyles.
Co-author Professor David Dinges said: "A take-away message from this line of research is the life-sustaining importance that healthy sleep duration and timing plays for everyone.
"As a global society, we need to re-evaluate how we view sleep as it relates to our overall health and ability to lead productive lives.
"Whether it is an astronaut being challenged to reach another planet or a newborn baby just learning to walk, the human body's need for sleep is as essential as our need for food and water and integral to our ability to thrive."