They can't put their head on a pillow unless they tie it to their cheek. They often sleep in a bag tied inside a compartment the size of a telephone booth. On early flights, astronauts worried that lack of air movement would cause them to suffocate on the carbon dioxide in their own exhalations. NASA put fans in the sleeping compartments, but they blew too much air up an astronaut's nose, preventing sleep. If the spacemen tried to sleep head down, the sleeping bags blew up like balloons.
Some astronauts now bungee themselves to a wall, or just float free in the cargo area.
Spaceflight also takes away the 24-hour cycle of night and day. Mir orbits Earth every 90 minutes, creating a confusion of sunrises and sunsets every 45 minutes. Such changes savage orientation, balance, sleep, and dreams. Sleep and dreams are controlled by nerves in the lower part of the brain, just above the spinal cord. These nerves are intimately connected with those involved in balance.