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Sleep international classification and sleep wake cycle

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"sleep-wake cycle" AND "circadian rhythm"

Sleep: International Classification and the Sleep-Wake Cycle

1. Classification Systems for Sleep Disorders

Three nosologies are used to classify sleep and sleep-wake disorders:
  1. International Classification of Sleep Disorders (ICSD-3) - published by the American Academy of Sleep Medicine (AASM), 3rd edition, 2014. It is the most comprehensive and precise system, preferred by sleep medicine specialists, and is the direct descendant of the first sleep disorders nosology commissioned in 1979 by the Association of Sleep Disorders Centers.
  2. DSM-5-TR (Diagnostic and Statistical Manual of Mental Disorders) - the standard used by general physicians/psychiatrists who are not sleep specialists. It organizes sleep-wake disorders into 12 main categories, with diagnostic criteria that are often identical or very similar to ICSD-3 for the major disorders.
  3. ICD-10/ICD-11 (World Health Organization) - used mainly for administrative coding, less popular clinically in the US.
(Kaplan & Sadock's Comprehensive Textbook of Psychiatry, p. 1349, 1153; Kaplan and Sadock's Synopsis of Psychiatry, p. 670)
ICSD-3 major diagnostic categories include:
  • Insomnia disorders (chronic, short-term, other)
  • Sleep-related breathing disorders (obstructive sleep apnea, central sleep apnea, sleep-related hypoventilation)
  • Central disorders of hypersomnolence (narcolepsy type 1/2, idiopathic hypersomnia, Kleine-Levin syndrome, insufficient sleep syndrome)
  • Circadian rhythm sleep-wake disorders (delayed/advanced sleep-wake phase, irregular sleep-wake rhythm, non-24-hour sleep-wake rhythm, shift work disorder, jet lag)
  • Parasomnias (NREM arousal disorders like sleepwalking/night terrors; REM parasomnias like REM sleep behavior disorder, nightmare disorder)
  • Sleep-related movement disorders (restless legs syndrome, periodic limb movement disorder, bruxism, rhythmic movement disorder)
  • Other sleep disorders
(Fishman's Pulmonary Diseases and Disorders, p. 1530; Scott-Brown's Otorhinolaryngology; Kaplan & Sadock's Comprehensive Textbook of Psychiatry Table 15-4)

2. The Sleep-Wake Cycle

Two states of sleep

Each night alternates between two physiologically distinct states (Guyton and Hall, p. 2206-2215):
  • NREM (non-REM/slow-wave) sleep: strong, low-frequency brain waves; the deep, restful sleep predominating in the first hours after sleep onset.
  • REM (paradoxical) sleep: rapid eye movements, active dreaming, near-total skeletal muscle atonia, and a desynchronized (fast, low-amplitude) EEG resembling wakefulness. REM occupies about 25% of total sleep time in young adults, recurring roughly every 90 minutes, with each episode lasting 5-30 minutes and lengthening as the night progresses.

Regulation - the two-process model

The timing and depth of the sleep-wake cycle are governed by the interaction of two processes:
  • Process C (circadian drive): generated by the suprachiasmatic nucleus (SCN) of the hypothalamus, the body's master clock, which runs on an intrinsic period slightly longer than 24 hours and is entrained daily by light input via the retinohypothalamic tract. The SCN drives the pineal gland's rhythmic secretion of melatonin, which rises in the evening, peaks at night, and falls before waking, acting back on SCN melatonin receptors (MT1/MT2) to reinforce the rhythm and promote sleep onset (Eric Kandel, Principles of Neural Science, p. 2833; Goodman & Gilman's Pharmacological Basis of Therapeutics, p. 4106; Stahl's Essential Psychopharmacology).
  • Process S (homeostatic sleep drive): sleep pressure that builds progressively during wakefulness (linked to adenosine accumulation) and dissipates during sleep.
Neurally, wake and sleep states are produced by mutually inhibitory ("flip-flop") circuits: ascending arousal systems (orexin/hypocretin neurons of the lateral hypothalamus, locus coeruleus noradrenergic neurons, raphe serotonergic neurons, tuberomammillary histaminergic neurons, and cholinergic brainstem/basal forebrain neurons) promote wakefulness, while the ventrolateral preoptic nucleus (VLPO) of the hypothalamus inhibits these arousal centers to permit NREM sleep onset. REM sleep is generated by reciprocal interaction of pontine cholinergic (REM-on) and monoaminergic (REM-off) neuron populations.

Clinical relevance

Disruption of the sleep-wake cycle underlies circadian rhythm sleep-wake disorders - e.g., in non-24-hour sleep-wake rhythm disorder the patient's clock runs on a 24.3-25 hour cycle instead of entraining to 24 hours, causing progressively shifting insomnia and daytime sleepiness; melatonin receptor agonists such as tasimelteon are used to help realign the cycle (Goldman-Cecil Medicine, p. 1439-1481; Harrison's Principles of Internal Medicine 22E, p. 2121).
A 2025 systematic review in Sleep Medicine Reviews (PMID: 41110263) examined how daylight saving/clock transitions disrupt sleep and sleepiness, reinforcing how sensitive the circadian sleep-wake system is to external timing cues - consistent with the SCN/melatonin mechanism above. A 2024 systematic review (PMID: 38713204) on prolonged-release melatonin in circadian medicine supports the textbook description of melatonin's role in resynchronizing the sleep-wake cycle.
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