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In space, no one can hear you snore - but that's probably because you're not asleep.

Most astronauts suffer serious levels of sleep deprivation that could be putting their lives in danger, a study has revealed.

The reasons for this are poorly understood, but the sun rising and setting every 90 minutes when orbiting Earth on the International Space Station (ISS) is thought to be one possible cause.

Day and night: Astronauts (Mike Hopkins is pictured) living on the International Space Station must contend with the bizarre notion of the sun rising and setting every 90 minutes as they orbit Earth. This, scientists believe, could be one of the reasons for the disturbed sleep patterns observed in the Harvard study

Scientists studied the sleep patterns of 64 astronauts on 80 space shuttle missions and 21 International Space Station (ISS) crew members before, during and after spaceflight.

ONE GIANT LEAF FOR MANKIND...

A functioning man-made leaf (pictured, right) that absorbs water and carbon dioxide to produce oxygen, just like natural leaves, has been created by an art graduate.

The new material could provide a steady source of oxygen for humans on long missions in space and even help us colonise new planets.

Designer Julian Melchiorri claims that the leaves they could be dumped on planets that humans will colonise, as 'real' plants cannot grow in zero gravity.

The material is made up of chloroplasts from plant cells that are suspended in a web of silk protein. The protein is extracted from natural silk fibres.

They found that on average astronauts get less than six hours sleep a night on orbiting space shuttles and just over six hours on ISS missions.

This was despite the American space agency Nasa scheduling 8.5 hours of sleep per night for space-travelling astronauts.

Around three-quarters of astronauts also resorted to sleeping pills during spaceflight, raising concerns about the effect the drugs may be having on their performance.

Dr Laura Barger, from Harvard Medical School in Boston who led the study reported in the journal The Lancet Neurology, said: 'Sleep deficiency is pervasive among crew members.

'It's clear that more effective measures are needed to promote adequate sleep in crew members, both during training and spaceflight, as sleep deficiency has been associated with performance decrements in numerous laboratory and field-based studies.'

She added: 'The ability for a crew member to optimally perform if awakened from sleep by an emergency alarm may be jeopardised by the use of sleep-promoting pharmaceuticals. Routine use of such medications by crew members operating spacecraft is of particular concern.'

Government health guidelines warned patients taking sleeping pills not to engage in hazardous occupations requiring a high degree of mental alertness and co-ordination, Dr Barger pointed out.

Scientists also believe that microgravity (water under microgravity conditions, with astronaut Chris Hadfield in the background, pictured) could be causing ISS inhabitants to lose valuable sleep and resort to sleeping pills

'This consideration is especially important because all crew members on a given mission may be under the influence of a sleep-promoting medication at the same time,' she said. 'In fact, on the four shuttle missions on which all crew members participated, all crew members reported taking sleep-medications on the same night 6 per cent of the time.'

With the sun rising and setting every 90 minutes, Earth orbit is known to be non-conducive to sleep.

The Apollo astronauts complained that their sleep was interrupted by light, noise and the cooling systems in space suits.

But sleep disturbance continues to affect astronauts despite the quiet and dark 'sleep stations' installed on the ISS, leading some scientists to speculate that microgravity itself may be to blame.

However the research showed that sleep deficiency began to build up in astronauts long before a mission launch.

On average, crew members enjoyed less than 6.5 hours sleep a night during training some three months before taking off into space - about half an hour less than the average American adult.

After missions ended however, at least half of the astronauts found they gained a great deal more sleep.

Just 12 per cent of sleep sessions on the space shuttle and 24 per cent on the ISS lasted seven hours or more, compared with 42 per cent and 50 per cent after the same crew members had finished their missions.

Astronauts' sleep patterns were studied using an actigraph, a device worn on the wrist which records sleep and wake cycles.

Researchers also looked at diary entries from astronauts recording alertness and sleep quality.

Co-author Dr Charles Czeisler, also from Harvard Medical School, said: 'Future exploration spaceflight missions to the moon, Mars, or beyond will require more effective countermeasures to optimise human performance by promoting sleep during spaceflight.

'These may include modifications to schedules, strategically timed exposure to specific wavelengths of light, and behavioural strategies to ensure adequate sleep, which is essential for maintaining health, performance and safety.'

Commenting on the findings in the journal, Dr Mathias Basner from the University of Pennsylvania, US, stressed the need for more research on the effects of space travel on sleep.

'Only four people have consecutively lived and worked for more than one year in space,' he said. 'As such, how sleep and behavioural health will be affected during space exploration is poorly understood.

'Studies of the physiology of sleep stages and the intensity of sleep.. in space are necessary to answer the important question of whether spaceflight reduces the need for sleep and therefore the ability to sleep, or whether it reduces the ability to sleep but not the need for sleep.'