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Sleeping in space requires that travelers sleep in a crew cabin, a small room about the size of a shower stall. They lie in a sleeping bag which is strapped to the wall. Astronauts have reported having nightmares, dreams, and snoring while sleeping in space.
Sleeping and crew accommodations need to be well ventilated; otherwise, astronauts can wake up oxygen-deprived and gasping for air, because a bubble of their own exhaled carbon dioxide had formed around their heads. Brain cells are extremely sensitive to a lack of oxygen and brain cells can start dying less than 5 minutes after their oxygen supply disappears; the result is that brain hypoxia can rapidly cause severe brain damage or even death. A decrease of oxygen to the brain can cause dementia and brain damage, as well as a host of other symptoms.
In the early 21st century, crew on the ISS were said to average about six hours of sleep per day.