National Sleep Foundation

Chapter 1: Normal Sleep

Stages of Human Sleep

What are the Stages of Human Sleep?

Following the discovery of rapid eye movement (REM) sleep in 1953,1 researchers learned that there are 3 basic states of consciousness: wakefulness, REM sleep, and non-rapid eye movement (NREM) sleep. NREM consists of 4 stages, each with its own typical EEG and physiologic pattern (See Figure 1.2 and Table 1.1). Although distinct sleep stages are defined for clarity of discussion and data management, there is a gradual merging from one stage into the other.

Figure 1.2: Characteristic electroencephalogram patterns of human sleep stages2

In Neurology, the EEG is used mainly to diagnose various brain abnormalities (e.g. foci of seizure discharges), in the sleep laboratory, two or three EEG channels are typically recorded mainly to ascertain whether the patient is awake or in what type or stage of sleep he/she is in. To that end, one is primarily interested in 4 forms of brain-waves (alpha, beta, delta, and theta) measured in cycles per second (cps). There are distinct EEG patterns for alert and relaxed wakefulness and each stage of sleep – stage 1, stage 2, and stages 3 and 4 (slow-wave sleep [SWS] and REM) (Table 1.1).

Table 1.1: Sleep stage characteristics3

  Brain Waves      
Sleep Stage Defining EEG Frequency [cps] Type Characteristics First Appearance % Sleep in a Young Adult Additional Comments
Alert wakefulness Fast, with many waves >13 Beta Low voltage, random pattern, with few rhythmic components      
Relaxed wakefulness 8-13 Alpha Low voltage, rhythmic alpha, with occasional bursts of the alertness pattern The person is relaxed or drowsy, with eyes closed.    
Stage 1 3-7 Theta Theta waves interspersed with brief periods of alpha waves As soon as alpha waves are < 50% of a 30-second epoch 2-5 Reactivity to outside stimuli diminished, while subjectively the sleeper may still feel awake.
Stage 2 12-14 lasting >0.5 seconds
Isolated slow/high amplitude waves
Sleep spindles
At least 1 sleep spindle or K-complex per 30 seconds on a stage 1 background When the first sleep spindle or K-complex appears 45-55 The most prominent sleep stage, deeper than stage 1, lighter than SWS
Stages 3 and 4 – slow wave sleep, (SWS, delta sleep) <4 Delta (<20% for stage 3, <50% for stage 4) High amplitude, low waves Occurs within 15-45 minutes after sleep onset 10-25 Duration of SWS depends on age (less in the elderly);
SWS is the deepest sleep of the night.
REM sleep Stage 1 pattern with “saw tooth waves” Low voltage, random, fast Eyes move; the autonomic system is activated (eg, respiratory and cardiac irregularities). First REM period occurs after ~85 minutes of NREM sleep. 20-25 A unique state, during which dreams usually occur. The brain is awake; the body is paralyzed (REM-related atonia).


  1. Aserinsky E, Kleitman N. Regularly occurring periods of eye motility, and concomitant phenomena, during sleep. Science. 1953;118:273-274.
  2. Hauri P. Current concepts: the sleep disorders. 2nd ed. Kalamazoo, Mich: The Upjohn Company; 1982.
  3. Rechtschaffen A, Kales A, eds. A manual of standardized terminology, techniques, and scoring system for sleep stages in human subjects. Los Angeles, Calif: Brain Information Service/Brain Research Institute, UCLA; 1968.