Sleep is divided into non-REM and REM stages. Non-REM is divided into N1, N2, and N3, with N1 being the transition from wakefulness to sleep, N2 characterized by sleep spindles and K-complexes, and N3 being slow wave sleep. REM sleep is where dreams occur and the eyes move rapidly. Sleep stages are classified using EEG waves, physiological markers, and subjective reports.
Sleep stages can be divided into two broad categories: non-REM sleep and rapid eye movement (REM) sleep. Non-REM sleep can be further divided into stages N1, N2 and N3. The way sleep scientists divide sleep into stages has changed several times since the categorization of sleep stages began in the late 1930s. The phases were first described in 1937 by Loomis et al, then updated in 1957 with the discovery of the Rapid Eye Movement, the deepest period of sleep, in which nearly all memorable dreams occur. An update in 2007, presented in The AASM Manual for the Scoring of Sleep and Associated Events, published by the American Academy of Sleep Medicine (AASM), merged the third and fourth stages of sleep, leaving only one third stage.
Sleep stages are classified using a variety of means, including measurement of EEG waves (commonly known as “brain waves”), physiological markers, and subjective reports. N1 is primarily defined as the transition of the brain from alpha waves (with a frequency of 8 to 13 Hz, common to awake people) to theta waves (with a frequency of 4 to 7 Hz). N1 is on the border between sleep and wakefulness, known as “drowsiness” or somnolence. Common to this stage are brief involuntary contractions known as hypnic jerks, as well as hypnagogic imagery, which can include phosphenes (fuzzy colored spots) or shape constants (patterned mental images).
The second of the sleep stages is N2. Characteristic signs of N2 sleep are “sleep spindles”, 0.5-1 second EEG jumps of 12-16 Hz waves and “K complexes”, short voltage spikes on the EEG lasting for a similar period of time and follow sleep spindles in bursts. K-complexes are known to occur randomly during N2 sleep, but they also occur in response to auditory stimuli, such as if someone knocks on a sleeping person’s door. During N2, conscious awareness of the external environment disappears and muscles relax. N2 makes up about half of sleep time in adults.
The third stage of sleep is N3, or slow wave sleep (SWS). Slow wave sleep is characterized by delta waves, (0.5 to 4 Hz), also called delta rhythms. Unusual sleep effects, called parasomnias, occur in this stage and include night terrors, bedwetting, sleepwalking, and sleep talking. When sleep deprived, a person falls into N3 and REM sleep very quickly, suggesting that both are essential.
The last stage of sleep, officially considered a part of N3, is the most famous stage of sleep, known as REM sleep. REM sleep is where dreams occur and the eyes move rapidly, possibly following dream objects. Most dreams are forgotten, even during REM sleep, but most people at least vaguely, if not vividly, remember the presence of dreams during REM sleep.
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