Wide Awake in Outer Space
On some space shuttle missions up to 50% of the crew take sleeping pills, and, over all, nearly half of all medication used in orbit is intended to help astronauts sleep. Even so, space travelers average about 2 hours sleep less each night in space than they do on the ground.
Above: In low-Earth orbit the Sun rises and sets every 90 minutes, which can be a sleep-scrambling experience for astronauts. The crew of shuttle mission STS-47 captured this picture of a dramatic sunrise -- one of many they enjoyed during their week-long mission.
That deficit adds up, says Dr. Ken Wright, Instructor in Medicine at Harvard Medical School, and Associate Neuroscientist at Brigham and Women's Hospital. "Research performed on Earth suggests that some of them, after a week or two on this restricted sleep schedule, are performing at the level of someone who's been awake for 24-plus hours.”
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On Earth, sleep can be disrupted by anything from a crying baby to tomorrow's exam. In space wakefulness can come from noise and excitement -- and, possibly, the disruption of the circadian rhythms that ensure a good night's sleep.
Sleep is, in large part, managed by our body's master clock, which is located in the brain's hypothalamus. This clock regulates the body's daily production of melatonin, a sleep-promoting hormone, and cortisol, a hormone that promotes wakefulness, and is also associated with stress. The clock also manages a multitude of other physiological cycles, including body temperature, growth hormone production, heart rate, and urine production. The circadian clock generates these cycles all on its own. But there's a problem. Free-running, the master clock produces cycles that average about 24.2 hours -- slightly longer than Earth's day. So the clock must be reset. It needs to be adjusted daily to ensure that the biological day and night don't get out of sync with the environment. On Earth, it's reset automatically, simply by our exposure to the high intensity light of day.
But in space, the right cue isn't so easily provided. The space shuttle orbits Earth every 90 minutes. So, instead of receiving the Terran pattern of 12 hours of light followed by 12 hours of dark, astronauts on the shuttle's flight deck experience 45 minutes of light succeeded by 45 minutes of dark. This rapid-fire exposure might disrupt -- or as sleep researchers say, "disentrain" -- the astronauts' circadian cycle.
It's important that the cycle be entrained: that the astronaut's biological day matches the environmental one. "The circadian system is set so that you're best prepared to be alert and awake during the biological day, and to sleep at [biological night],” says Wright. If the body is required to perform activities at the wrong biological time, he explains, it will not function optimally.
For that reason NASA sometimes deliberately shifts the astronauts' cycles before sending them into space, making sure that their biological day coincides with the crucial period of launch, according to Dr. Bette Siegel, a scientist in the Bioastronautics division at NASA headquarters. Effecting the shift is easy: astronauts are exposed to high intensity light at key times for three to ten days before liftoff. By the time the shuttle is ready to leave Earth, the crew is bright-eyed and alert.
Once in orbit the biological clocks of astronauts might need to be adjusted further to align with another critical time -- the moment of landing. It's done by requiring the crew to wake up earlier and earlier each day.
Right: An "actiwatch," worn by astronauts to document sleep patterns in space. Credit: Brigham and Women's Hospital, Harvard Medical School, NASA Ames, and UC San Diego.
But even with this information, scientists still need to answer some basic questions in order to develop countermeasures against unwanted wakefulness. For instance, what exactly controls the master clock? What intensity of light will trigger it -- and which colors? Does gravity itself provide a cue? All these questions will grow in importance as humans move farther into space.
Take the exploration of Mars, for example. On Mars, daylight is primarily yellowish-brown. On Earth, it's blue-green. How will the human clock respond to the unearthly color of Martian skies? Some research indicates that it could make a difference. Melatonin production, for example, is suppressed more by some wavelengths of light than by others.
Below: The unearthly color of Martian daylight could scramble the circadian rhythms of human explorers. Artist Pat Rawlings painted this beautiful scene. [more]
Learning to manage the circadian clock is critical to exploring space. But astronauts are hardly the only ones with sleep problems.
"The space environment," says Wright, "provides us with a unique opportunity to understand something more about the functions of sleep." Part of the research involves trying to understand ways to promote wakefulness for shift workers, or people suffering jet lag, or simply for the many people who don't get enough sleep.
"It's an exciting topic,” says Wright, "because it affects everyone.” Indeed, countermeasures that Wright and his colleagues devise for astronauts in space might well provide a more restful night for those of us remaining on Earth.
Sleeping Better in Space -- learn more about NASA's sleep studies and clinical trials of melatonin as a hypnotic. From Spaceflight.nasa.gov and the NeuroLab Team.
Sleeping Bags in Space -- Visit this web page from Spaceflight.NASA.gov to learn more about sleeping bags and rigid sleep stations on NASA's space shuttle.
Bedtime on Columbia -- Astronaut Richard M. Linnehan, payload commander, is in the sleep compartment, which is located in the mid deck of the Space Shuttle Columbia. A photo from spaceflight.nasa.gov
Melatonin Mania -- a brief discussion of melatonin from Scientific American (external site).
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