National Sleep Foundation

Chapter 1: Normal Sleep

Stages of Human Sleep

Following the discovery of rapid eye movement (REM) sleep in 1953, researchers learned that there are three basic states of consciousness: wakefulness, REM sleep, and non-rapid eye movement (NREM) sleep.16(See Table 1.1.)

REM sleep is “an active period of sleep marked by intense brain activity. Brain waves are fast and desynchronized, similar to those in the waking state. Breathing becomes more rapid, irregular, and shallow; eyes move rapidly in various directions and limb muscles become temporarily paralyzed. Heart rate increases and blood pressure rises. This also is the sleep stage in which most dreams occur.”17

REM sleep is thought to play a role in memory consolidation, the synthesis and organization of cognition, and mood regulation.18 Depriving someone of just REM sleep (by waking the subject upon when he or she enters into REM, but allowing NREM to occur) results both in the person making increasingly frequent attempts to enter REM sleep and spending increased time in REM sleep.

NREM sleep is characterized by a reduction in physiological activity. As sleep deepens, a person’s brain waves slow down and gain amplitude, both breathing and the heart rate slow down, and the individual’s blood pressure drops.

NREM sleep consists of three stages:19

·         N1 (formerly “stage 1”) is a time of drowsiness or transition from being awake to falling asleep. Brain waves and muscle activity begin slowing down in this stage. People in N1 sleep may experience sudden muscle jerks, preceded by a falling sensation.

·         N2 (formerly “stage 2”) is a period of light sleep during which eye movements stop. Brain waves become slower, with occasional bursts of rapid waves (called sleep spindles) and spontaneous periods of muscle tone mixed with periods of muscle relaxation. The heart rate slows and body temperature decreases.

·         N3 (formerly “stages 3 and 4”) is called “slow wave sleep” (SWS) and is characterized by the presence of slow brain waves called “delta waves” interspersed with smaller, faster waves. Blood pressure falls, breathing slows, and temperatures drops even lower, with the body becoming immobile. Sleep is deeper, with no eye movement and decreased muscle activity, although muscles retain their ability to function. It is most difficult to be awakened during SWS, and people may feel groggy or disoriented for several minutes after they wake up from this stage. During SWS, some children experience bedwetting, night terrors, or sleepwalking.  SWS seems to be associated with bodily recovery, certain types of learning,20 and central nervous system changes.21 The amount of SWS a person gets is directly related to accumulated sleep need — the longer a person has been awake, the more SWS he or she gets when sleep occurs.

Although the role each of these states plays in overall health is uncertain, having the right balance between them is believed to be important for obtaining restful, restorative sleep and for promoting processes such as learning, memory, mood, and ability to concentrate.22 No one sleep state is more important than the other, just as no one organ in your body is more important than the other.  Each is necessary for life, as deprivation of any one of those stages has been shown to reduce health and longevity in laboratory animals.23There are distinct and typical electroencephalogram (EEG) and physiologic patterns for alert and relaxed wakefulness, for REM sleep, and for each of the stages of NREM. (See Figure 1.1.) Although the sleep stages are specifically defined, in reality, they gradually merge from one to another.

Figure 1.1: Characteristic electroencephalogram patterns of human sleep stages.24

In the sleep laboratory, two or three EEG channels are typically recorded, mainly to determine whether a patient is awake and, if not, what sleep stage he or she is in. The EEG captures the four types of brain waves that occur during wakefulness and sleep, which are measured in cycles per second (cps):25

·         Beta waves occur during daily wakefulness. They have the highest frequency and the lowest amplitude, compared to other waves. These patterns also show a lot of variability.

·         Alpha waves occur during wakefulness and periods of relaxation (i.e., during meditation). These waves are slower, and have less amplitude and variability than beta waves. 

·         Theta waves occur during stages 1 and 2 and are slower in frequency and greater in amplitude than alpha waves. As a person moves from N1 to N2 sleep, theta wave activity continues; every few minutes, sleep spindles (sudden increase in wave frequency) and K-complexes (sudden increase in wave amplitude) occur.

·         Delta waves occur during N3 sleep and are the slowest waves with the highest amplitude. Delta sleep is the deepest sleep.

Table 1.1: Sleep stage characteristics26


Sleep Stage


Brain Waves



First Appearance






Defining EEG
Frequency  [cps]







Alert wakefulness


Fast, with many waves >13 cps



Low voltage, random pattern, with few rhythmic components



Relaxed wakefulness


8-13 cps



Low voltage, rhythmic alpha, with occasional bursts of the alertness pattern


The person is relaxed or drowsy, with eyes closed.





3-7 cps



Theta waves interspersed with brief periods of alpha waves


As soon as alpha waves are < 50% of a 30-second epoch

Reactivity to outside stimuli diminishes; sleepers may still feel awake



12-14 lasting >0.5 seconds.

Isolated slow / high amplitude


Theta with

sleep spindles

and K-complexes

At least 1 sleep spindle or K-complex per 30 seconds on a N1 back-ground


When the first sleep spindle or K-complex appears

The most prominent sleep stage, deeper than N1 sleep, lighter than SWS


N3 – Slow Wave Sleep (SWS)


< 4 cps




High amplitude, low waves


Usually occurs within 15-45 minutes after sleep onset

Deepest sleep; duration of SWS depends on age (less in the elderly)


REM sleep

N1 pattern with “saw tooth waves”

Low voltage, random, fast

Eyes move; the autonomic system is activated (e.g., respiratory and cardiac irregularities).


First REM period occurs after ~85 minutes of NREM sleep.

A unique state, in which dreams usually occur. The brain is awake & body paralyzed (REM-related atonia).








16. Aserinsky E, Kleitman N. Regularly occurring periods of eye motility, and concomitant phenomena, during sleep. Science. 1953;118:273-274

17.  National Sleep Foundation, The Sleep-Wake Cycle: Its Physiology and Impact on Health, Arlington: NSF, 2006. Available at:

18. Bonnet MH. Acute sleep deprivation. In: Kryger MH, Roth T, Dement WC, eds. Principles and Practice of Sleep Medicine. 5th ed. Philadelphia: Elsevier Saunders; 2011:54-66.

19.  National Sleep Foundation, The Sleep-Wake Cycle: Its Physiology and Impact on Health, Arlington: NSF, 2006. Available at:

20. Tononi G, Cirelli C. Sleep function and synaptic homeostasis. Sleep Med Rev. 2006;10:49-62.

21. Huber R, Esser SK, Ferrarelli F, et al. TMS-Induced cortical potentiation during wakefulness locally increases slow wave activity during sleep. PLoS ONE 2(3):e276. doi:10.1371/journal.pone.0000276.

22.  National Sleep Foundation, The Sleep-Wake Cycle: Its Physiology and Impact on Health, Arlington: NSF, 2006. Available at:

23. Bonnet MH. Acute sleep deprivation. In: Kryger MH, Roth T, Dement WC, eds. Principles and Practice of Sleep Medicine. 5th ed. Philadelphia: Elsevier Saunders; 2011:54-66.

24. Hauri P. Current Concepts: The Sleep Disorders, 2nd Edition, Kalamazoo, MI: Upjohn Company; 1982.

25. Missouri University of Science, Psychology World website, Sleep Stages, no date. Available online at:

26. Rechtschaffen A, Kales A, eds. A manual of standardized terminology, techniques, and scoring system for sleep stages in human subjects. Los Angeles: Brain Information Service/Brain Research Institute, UCLA; 1968.