What is sleep?

Sleep is a physiological state in which the threshold for outside stimuli is high. Unlike in coma or other such states, it is rapidly reversible.

The stages of sleep can be seen on an EEG:

REM: shows up as a “sawtooth” pattern made of low voltage alpha-beta waves.


– Stage I: Low voltage, vertex sharp waves

– Stage II: K complex; sleep spindle

– Stage III: Slow-waves (more than 20%, less than 50%)

– Stage IV: Slow-waves (more than 50%)

Some important sleep parameters

Sleep latency: the time that passes between the start of the sleep recording and the onset of sleep.

Onset of sleep: Stage I or the first 3 successive epochs during deep sleep.

REM latency: The time that passes between the onset of sleep and the appearance of the first REM epoch.

Certain points in the physiology of sleep

Autonomic System: During NREM the parasympathetic tone increases. During phasic REM the sympathetic tone increases.

KVS: During NREM, KB and KR decrease. During REM arrhythmia occurs.

Pulmonary System: SS is reduced.

Thermoregulation: During NREM, the body cools down and during REM it warms up.

Endocrine changes: During SWS, GH and prolactin is secreted. Sleep inhibits TSH. Abnormalities appear in the HPA.

Sex: During REM, penile and clitoral erections occur.

Neurobiology of sleep and wakefulness

– Anatomy: The main centre of sleep and wakefulness is the RAS.

– Biochemistry of sleep and wakefulness:

NE (norepinephrine) and serotonin levels in LC (an area of the brain called locus coeruleus) are very high in wakefulness, low during NREM, and absent during REM. 5HT (serotonin) plays a role in sleep induction.

Ach (acetylcholine): Very high during REM and in wakefulness. REM is inductive.

Histamine: High in wakefulness. Inhibited during sleep.

DA (dopamine): Increased in wakefulness.

Orexine (hypocretin): Increased in wakefulness, reduced in narcolepsy.