text: Studies of human sleep have established five well-defined stages, according to electroencephalographic (EEG) recordings and polysomnography:\n\n    * Non-REM sleep (NREM), accounts for 75-80% of total sleep time:\n          o Stage 1, with near-disappearance of the alpha waves seen in awake states, and appearance for the first time of theta waves. The stage is sometimes referred to as somnolence, or "drowsy sleep". It appears at sleep onset (as it is mostly a transition state into Stage 2), and can be associated with so-called hypnagogic hallucinations. In this period, the subject loses some muscle tone, and conscious awareness of the external environment: Stage 1 can be thought of as a gateway state between wake and sleep.\n          o Stage 2, with "sleep spindles" (12–16 Hz) and "K-complexes". The EMG lowers, and conscious awareness of the external environment all but disappears. This occupies 45-55% of total sleep.\n          o Stage 3, with delta waves, also called delta rhythms (1–2 Hz), is considered part of slow-wave sleep (SWS) and functions primarily as a transition into stage four. Overall it occupies 3-8% of total sleep time.\n          o Stage 4 is true delta sleep. It predominates the first third of the night and accounts for 10-15% of total sleep time. This is often described as the deepest stage of sleep; it is exceedingly difficult to wake a subject in this state. This is the stage in which night terrors and sleepwalking occur.\n\n    * Stage 5, or Rapid eye movement (REM) sleep, associated with dreaming, especially bizarre, visual, and seemingly random dreams. REM sleep is predominant in the final third of a sleep period, its timing linked to circadian rhythm and body temperature. The EEG in this period is aroused and looks similar to stage 1, and sometimes includes beta waves.\n\nSleep proceeds in cycles of NREM and REM phases. In humans, the cycle of REM and NREM is approximately 90 minutes. Each stage may have a distinct physiological function. Drugs such as alcohol and sleeping pills can suppress certain stages of sleep (see sleep deprivation below). This can result in a sleep that exhibits loss of consciousness but does not fulfill its physiological functions.\n\nEach sleep stage is not necessarily uniform. Within a given stage, a cyclical alternating pattern may be observed.