What Are the Stages of Sleep? Four States Cycling Through the Night
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Sleep is not one condition that deepens and lightens. It is a sequence of distinct states with different brain activity, different muscle tone and different functions, cycling roughly every ninety minutes, and the proportions shift across the night so that the first half and the second half of a night's sleep do quite different jobs.
The stages
Sleep divides into rapid eye movement sleep and three stages of non-rapid eye movement sleep, distinguished by brain wave patterns recorded on an electroencephalogram:
- •Stage one, the transition from wakefulness, lasting a few minutes, with slowing brain activity, frequently accompanied by the falling sensation and a sudden muscle jerk
- •Stage two, which occupies the largest share of total sleep, marked by brief bursts of activity called sleep spindles and by sharp waveforms called K-complexes, both of which have been linked to memory processing and to protecting sleep from disturbance
- •Stage three, deep or slow wave sleep, with large slow brain waves, from which waking is difficult and leaves a person confused, and which is when growth hormone is released and physical restoration is concentrated
- •Rapid eye movement sleep, with brain activity resembling wakefulness, rapid eye movements, irregular breathing and heart rate, and near-complete paralysis of the skeletal muscles
- •Cycles of roughly ninety minutes repeat four to six times a night
- •The proportions shift, with deep sleep dominating early cycles and rapid eye movement periods lengthening towards morning, which is why a shortened night disproportionately removes dream sleep
Why the paralysis matters
During rapid eye movement sleep the brain actively inhibits motor neurons, producing atonia in which the skeletal muscles are effectively disconnected, while the muscles controlling breathing and the eyes continue. The function is straightforwardly protective, since the motor cortex is highly active during dreaming and without inhibition a sleeper would act out the dream. That is exactly what happens in rapid eye movement sleep behaviour disorder, in which the paralysis fails and people physically enact dreams, sometimes injuring themselves or others, and which is significant medically because it frequently precedes Parkinson's disease and related conditions by years. The same mechanism explains sleep paralysis, which occurs when a person becomes conscious while the atonia persists, producing an inability to move that is frequently accompanied by a sensation of pressure on the chest and a vivid sense of a presence in the room, a combination that appears across cultures with locally specific interpretations.
What each stage appears to do
Functions are still being worked out and several are well supported. Deep slow wave sleep is associated with physical restoration, immune function and, importantly, with memory consolidation of factual material, with evidence that information learned during the day is replayed during slow wave sleep and transferred towards long-term storage, and that experimentally enhancing slow waves improves recall. Rapid eye movement sleep is associated with procedural and emotional memory, with creative recombination of material, and with processing the emotional charge attached to experiences, which is one explanation for why a difficult event feels less acute after sleeping. A discovery in 2013 identified increased clearance of metabolic waste from brain tissue during sleep, through a system of fluid channels, including proteins associated with neurodegenerative disease, which supplies a plausible physical reason why sleep is not optional. Dreaming occurs in all stages and is most vivid and narrative during rapid eye movement sleep.
What disrupts the architecture
Many things damage the structure of sleep without necessarily reducing its duration, which is why total hours is an incomplete measure. Alcohol suppresses rapid eye movement sleep early in the night and produces rebound and fragmentation later, which is why sleep after drinking is unrefreshing despite feeling immediate. Sleep apnoea, in which the airway repeatedly closes, causes hundreds of brief arousals that the sleeper does not remember, preventing progression into deep stages and producing daytime sleepiness that a full night in bed does not resolve. Ageing reduces slow wave sleep substantially, which is a robust finding and is implicated in age-related memory change. Shift work and jet lag misalign sleep with the circadian signal, so sleep occurs at a time when the body is promoting wakefulness and is shorter and lighter as a result. Several common medications, including some antidepressants, suppress rapid eye movement sleep. And irregular timing appears to matter independently of duration, with consistency of schedule emerging in recent research as a strong predictor of health outcomes.
The takeaway
Sleep cycles roughly every ninety minutes through three non-rapid eye movement stages and rapid eye movement sleep, with deep slow wave sleep dominating early cycles and dream sleep lengthening towards morning. Muscle paralysis during dream sleep prevents acting out dreams, and its failure or persistence explains dream enactment disorder and sleep paralysis. Deep sleep supports factual memory consolidation and waste clearance from brain tissue, while dream sleep supports procedural and emotional processing.