Sleep is a reversible, naturally recurring state of reduced responsiveness and altered consciousness, distinct from unconsciousness in that a sleeper can be aroused. Modern sleep science divides sleep into two main types: REM (rapid eye movement) and NREM (non-REM). NREM is further subdivided into three stages. N1 is the lightest stage, marking the brief transition from wakefulness. N2 is light sleep characterized by sleep spindles and K-complexes, and it makes up roughly 45 to 55 percent of total sleep time. N3, also called slow-wave sleep, is the deepest and most physically restorative stage.
REM sleep is famously paradoxical: the brain shows activity similar to wakefulness, the eyes move rapidly, and vivid dreaming occurs, yet the body is paralyzed by a state called atonia, which prevents the sleeper from physically acting out dreams. Because the brain is active but the body immobile, REM is sometimes called paradoxical sleep.
A complete sleep cycle progresses through the NREM stages and then into REM, lasting approximately 90 minutes. Adults typically experience four to six such cycles per night. Sleep architecture, the structure and pattern of these stages across a night, shifts as the night progresses: early cycles contain more deep N3 sleep, while later cycles contain proportionally more REM. This dynamic organization reflects the brain's differing needs for physical restoration early in the night and for memory, emotional processing, and dreaming later on.