John Glenn decked out in sleep-monitoring equipment, stands near his sleep station on the middeck of the Earth-orbiting space shuttle Discovery in 1998.
Things like motion sickness and the very excitement of being in space can cause disruptions to an astronaut's sleep patterns. The accommodations are tight, and the crew can easily hear one another. And then there's the frequent rising of the Sun: every 90 minutes during a mission. Some astronauts use sleep masks to shield their eyes from the sunlight coming in through the windows.