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NASA-supported sleep researchers are learning new and surprising things about naps.

Space travel is sleepless work.

Despite NASA recommendations that astronauts sleep 8 hours a day, they usually don't. Strange sights and sounds, the stress of riding a powerful rocket, the lack of a normal day-night cycle--all these things tend to keep space travelers awake. Studies show that astronauts typically sleep 0.5 to 2.5 hours less than they do on Earth.

Astronaut upside down

Right: Could you sleep like this? [More]

Although many astronauts report feeling fully rested after only six hours of sleep, the fact is, sleeplessness can cause irritability, forgetfulness and fatigue--none of which astronauts need to deal with while piloting complicated 'ships that hurtle through space at tens of thousands of miles per hour.

The solution seems simple: Take a nap.

But naps are a double-edged sword. Sometimes napping can leave you feeling even drowsier than before. If your body enters a deep sleep, trying to wake after only an hour or so can be very unpleasant, and you might remain groggy for some time afterward. This is called "sleep inertia."

Why do naps sometimes backfire? Researchers don't yet know the physical causes of sleep inertia, but they would like to be able to predict, at least, when it's going to strike. This could help doctors prescribe naps of the right time and duration for drowsy people in high-risk professions.

Helping astronauts nap was the goal of a recent series of experiments funded by NASA in cooperation with the National Space Biomedical Research Institute. In those experiments, led by David Dinges, a professor at the University of Pennsylvania School of Medicine, 91 volunteers spent 10 days living on one of 18 different sleep schedules, all in a laboratory setting. The sleep schedules combined various amounts of "anchor sleep," ranging from about 4 to 8 hours in length, with daily naps of 0 to 2.5 hours.

To measure how effective the naps were, the scientists gave the volunteers a battery of tests probing memory, alertness, response time, and other cognitive skills throughout the experiment. They also measured things like core body temperature and hormone levels in blood and saliva, all of which fluctuate in a natural daily cycle known as a person's "biological clock."

In general, they found, longer naps were better. No surprise there. But they also found that some cognitive functions benefited more from napping than others:

"To our amazement, working memory performance benefited from the naps, [but] vigilance and basic alertness did not benefit very much," says Dinges.

Shuttle cockpit

Above: Shuttle pilots need to be mentally sharp to operate controls like these. [More]

"Working memory," he explains, "involves focusing attention on one task while holding other tasks in memory ... and is a fundamental ability critical to performing complex work [like piloting a spaceship]. A poor working memory could result in errors."

For vigilance and alertness, which involve the ability to maintain sustained attention and to notice important details, they found that the total amount of sleep during 24 hours remained the most important factor.

Another interesting finding was that naps didn't work as well for volunteers on a nocturnal schedule. Sleep schedules for some of Dinges' subjects were flipped, so that anchor sleep occurred when their bodies thought it was daytime. The nap, then, fell in the middle of biological nighttime. This simulated what might happen when an astronaut's biological clock is out of sync with the mission schedule.

Dr. David Dinges

These out-of-sync volunteers had a very hard time waking from naps, and the grogginess of sleep inertia lasted for up to an hour. Some sleep inertia did occur after naps on a normal schedule too, notes Dinges, but the inertia after a nighttime nap was much more severe.

Right: Dr. David Dinges, University of Pennsylvania. [More]

The ultimate goal, says Dinges, is to tie all these data together into a mathematical model of naps. Such a model, written as a computer program, could prescribe effective naps compatible with the scheduling demands of a mission. Not only astronauts would benefit from such a program, but also doctors, pilots, firefighters ... the list goes on.

Such a program is still in the future. Meanwhile, Dinges notes another finding of their study: Naps are a short-term fix, offering only temporary boosts in mental acuity. "They cannot replace adequate recovery sleep over many days," he says.

In the end, there's no substitute for 8 sweet hours of shut-eye.

More Information
Wide Awake in Outer Space -- (Science@NASA) NASA researchers are exploring ways to help astronauts get a better night's sleep.

"Despite NASA recommendations that astronauts sleep 8 hr per day in space, sleep is typically restricted in space flight, averaging 0.5 hr to 2.5 hr below astronauts' ground-based sleep durations," says Dr. David Dinges. "Six studies over 25 years have documented average daily sleep durations in space flight are between 4.0 hr and 6.5 hr; and acute total sleep loss (24-36 hr without sleep) can also occur prior to critical operations."

Space-sleep research references.

See also, Mallis M.M., Deroshia C.W., Circadian rhythms, sleep, and performance in space. Aviation, Space and Environmental Medicine 2005; 76(6, Suppl.), B94-107

Sleeping in Space -- more information from the Canadian Space Agency

Facts about Sleep and Fatigue -- from the Roads and Traffic Authority of Australia
Feature Author: Patrick L. Barry, Dr. Tony Phillips
Feature Production Editor: Dr. Tony Phillips
Feature Production Credit: Science@NASA