Stages of Sleep

Sleep staging is done via an overnight sleep study or polysomnogram (PSG) that includes, at a minimum, EEG, an electro-oculogram (looking at eye movement), and an electromyelogram (looking at skeletal muscle movement, usually on chin). Most PSGs have additional leads to examine limb movement and respiration. As of 2007, the American Academy of Sleep Medicine recognized 3 non-REM sleep stages, plus REM:


On EEG, predominantly low amplitude, relatively fast alpha activity (8-13 waves/sec) during relaxation or mixed alpha and beta activity (>13 waves/sec) accompanied by fast eye activity during vigilant/alert wakefulness.

N1 (Non-REM stage 1)

Very light sleep. EEG slows down, but remains low amplitude. Theta waves appear (medium amplitude, 4-7 waves/sec). Stage 1 is usually brief, and is a transition between wakefulness and Stage 2.

N2 (Non-REM stage 2)

EEG continues to show predominantly theta activity, with the introduction of sharp waveforms called K-complexes and brief bursts of beta activity called sleep spindles.

N3 (Non-REM stage 3)

“Slow Wave Sleep”, incoporates what had previously been called Stages 3 and 4 NREM. Deep sleep. EEG reflects high amplitude, low frequency delta waves (< 4 waves/sec). Most parasomnias (sleepwalking, night terrors) arise from N3 sleep.

REM (Rapid eye movement)

Sometimes called “paradoxical sleep” because the EEG looks like wakefulness (quick, mixed frequency, low amplitude) but the person is asleep. Can be distinguished from wakefulness by evidence of rapid eye movements and very low skeletal muscle tone. Most dreaming (and nightmares) occur during REM.

Other info about sleep stages:

  • Muscle tone & activity is greatest in wakefulness, declines in stages N1 – N3, and lowest during REM
  • At birth, roughly half of sleep is REM
  • With age, slow-wave and REM sleep diminishes, leaving mostly light sleep. Most rapid decline in REM occurs during early childhood. Most rapid decline in N3 (slow-wave sleep) occurs during adolescence. However, general trend towards decline evident throughout development.
  • In a healthy young adult, slow-wave sleep occurs mostly during the first ½ of the night, REM mostly during the second ½. Most of sleep is spent in stage 2 (about 50%), REM (about 25%), or Stage 4 (about 10-15%).

Important points for neuropsychologists

  • Adequate sleep quantity and quality are essential for daytime functioning, with the most marked effects of poor sleep seen in sustained attention/vigilance, coordination, and emotion regulation. There can also be effects on memory retrieval and aspects of executive functioning (EF). Finally, emerging evidence suggests that sleep is essential to long-term memory consolidation as well.
  • Sleep generation and regulation is dependent on a broad system that includes the brainstem through the diencephalon and limbic system. Any neurological condition or injury that affects a portion of this system can impact sleep. Dementing conditions are often accompanied by sleep disturbances, as is Parkinson’s Disease.
  • Many psychotropic meds can affect sleep. If sleep is a complaint, consider whether meds are contributing.
  • Sleep is particularly important in the epilepsies, as sleep deprivation can promote seizures, and seizures can occur during sleep (in some cases, sleep is the time they occur most).
  • The sleep-wake cycle is influenced by two processes:
    • Time spent awake relative to time recently spent asleep. During wakefulness there is a build-up of sleep pressure, roughly reflected in basal forebrain adenosine levels, that quickly dissipates during sleep, especially N3 (slow wave) sleep. In healthy sleepers, sleep pressure is greatest just prior to sleep onset and, of the two processes, tends to be the major contributor to sleep onset.
    • The circadian rhythm, which is an endogenous rhythm which roughly approximates 24 hrs in healthy sleepers, and which is entrained to a 24-hr cycle mostly by bright light exposure. A major marker, core body temp, is usually lowest at 2-4 am, highest in late afternoon. Consequently, this rhythm helps sustain wakefulness even as sleep pressure builds, and sustains sleep even as sleep pressure dissipates.
  • There are over 80 different sleep disorders. Two of the most important to know as a neuropsychologist are:
    • Narcolepsy: Misalignment of the various aspects of sleep. For example, aspects of REM can intrude into wakefulness during cataplexy (sudden loss of muscle tone during wakefulness, often when excited) or occur right around sleep onset or offset (hypnagogic and hypnopompic hallucinations, sleep paralysis in which person is awake but unable to move). There is also excessive daytime sleepiness, which is treated with stimulants.
    • Obstructive Sleep Apnea: Repetitive obstruction and/or collapse of the airway during sleep, especially seen during REM sleep, conventionally defined as occurring > 10 times/hour in adults or >5 times/hour in children. Results in frequent brief arousals and intermittent hypoxia, daytime sleepiness, poor daytime vigilance, motor uncoordination, depression, and ADHD symptoms in young kids. May cause memory and EF deficits, too.