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."