It’s hard to sleep in outer space. On the International Space Station (ISS), the sun rises every 90 minutes when the station circles Earth. Space suits can be uncomfortable, too: After landing on the moon in 1969, Buzz Aldrin reported getting only “a couple of hours of mentally fitful drowsing” due to the noise and the cold. Now, a new study published online today in The Lancet Neurology shows the extent of sleep deprivation among astronauts. Researchers tracked the sleep patterns of 85 crew members aboard the ISS and space shuttle and found that despite an official flight schedule mandating 8.5 hours of sleep per night, they rarely got more than five. In fact, getting a full night’s rest was so difficult that three-quarters of shuttle mission crew members used sleep medication, and sometimes entire teams were sedated on the same night. Although, unlike astronauts from Aldrin’s day, crew members now sleep in quiet, dark chambers, lack of gravity itself may contribute to the problem. Given that sleep deprivation contributes to up to 80% of aviation accidents, it’s important to better understand why sleep is so difficult in space, the authors say.