Sleeping in microgravity can be challenging. In the weightless environment of the International Space Station (ISS), astronauts can't lie down to sleep: there is no real "up" or "down.” Astronauts go to bed in their sleep stations — personal sleep compartments the size of a telephone booth that have a sleeping bag, a pillow, a lamp, an air vent, a personal laptop, and a place for personal belongings. In the weightless environment of space, the carbon dioxide (CO2) that astronauts expel could form a bubble around their head. That's why they have to sleep near an air vent. Crew members who want to sleep outside the sleep compartments can secure their sleeping bag to the floor, the ceiling, or the wall. They generally use earplugs and a sleep mask to block out the noise and light. Even though astronauts are allotted about 8½ hours for sleep every day, many of them have reported needing only about 6 hours to feel fully rested. Some specialists believe that this is because the body tires less quickly in weightlessness due to the muscles not having to work as hard as they do on earth. While orbiting the earth, astronauts witness 16 sunrises and sunsets every 24 hours. While seeing a sunrise every 90 minutes may seem like an incredible experience, it can also make it difficult for astronauts to maintain a regular sleep pattern. Things also change once they get home. After a long-duration stay in space, some astronauts have reported the sensation of floating over their mattress for a few days after their return to earth.
