Astronaut sleeping on ship

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Astronaut Norm Thagard takes an uncomfortable nap aboard the Russian Mir Space Station in 1995.

Although they may have other talents, few astronauts are much good at sleeping in space. NASA researchers have been studying the problem by hooking up space shuttle crews to all sorts of sleep-monitoring devices. The mystery remains unsolved, but one surprising discovery has been made: There’s virtually no snoring in space.

G. Kim Prisk, a medical professor at the University of California, San Diego, explains that earthly snoring occurs when “gravity pulls the tongue and soft tissues in the rear of your mouth backward. If your airway is partially closed, you get these tissues flapping.” In space, the body’s fluids move toward the head, says Prisk, “so you’d expect their throats would swell and obstruct their breathing. But that doesn’t happen. Whatever it is that makes them lose sleep, it’s not upper airway obstruction.”

Researchers now suspect the answer may lie in the astronauts’ circadian rhythms, or other brain activities.