Sleep was, in "every sentient being in the universe", an essential (TV: Sleep No More) periodic state of physiological rest during which the body was not active and the mind was unconscious. Both the human and Gallifreyan species slept with their eyes closed. However, while humans normally required a full night's sleep, Gallifreyans could make do with as little as an hour. (PROSE: The Highlanders)
Although a sleeping individual appeared entirely inactive, some functions of his or her brain were actually more active than during wakefulness. The high amplitude brain waves known as delta waves were associated with sleep. (TV: Kinda) When sleeping, series of thoughts, images, or emotions known as dreams could occur. (TV: The Time Monster)
During a certain stage of sleep, humans experienced an almost complete paralysis of the body, which prevented them from getting hurt while acting out the movements occurring in their dreams. (PROSE: Dry Pilgrimage)
Sleep occurred naturally, but could also be induced through soporific substances, (TV: The Name of the Doctor) technological augmentation of the delta waves, (TV: Kinda) or hypnosis. (TV: The War Machines)