Space travel can be exciting - and restless! NASA researchers are exploring ways to help astronauts enjoy a better night's sleep on the space station... and beyond.

By Karen Miller

Astronauts sleep poorly in space, and it's no wonder. Just consider: the excitement of blasting off on a powerful rocket, the strange sensations of floating in free-fall, the novelty of mornings that return every 90 minutes... Who could sleep through all that?

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 travellers average about 2 hours sleep less each night in space than they do on the ground.

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.”

Fatigue, on Earth or in space, is a serious problem. It affects performance, increasing irritability, diminishing concentration, and decreasing reaction time. And it increases the risk of accidents.

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.

Researchers still aren't sure exactly what happens to the circadian clocks of astronauts under such circumstances. To help find out, astronauts wear a wrist device that tracks astronaut sleep patterns along with their light exposures. "We have models,” says Wright, "where we can take their sleeping history and their light exposure history and predict what's going to happen to their [internal] clock.” The device, along with sleep diaries kept by the astronauts, will help researchers figure out which factors - such as light exposure, temperature, or ambient noise in the close confines of a spacecraft - affect sleep most during spaceflight.

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 colours? 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 colour 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.

Artist Pat Rawlings painted this beautiful scene. [more] The unearthly colour of Martian daylight could scramble the circadian rhythms of human explorers.

 
Of more concern, perhaps, is the length of the Martian day: 24 hours and thirty-nine minutes. "That is significantly different than the period of the clock in humans,” notes Wright. It's possible, he says, that the human clock might not be able to adapt to Mars. Ongoing research addresses this question by exploring countermeasures - for example, different patterns of light exposure - that will entrain the human clock to a longer day.

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.