Sleep disorders are not all variations on one theme. Each major sleep disorder breaks a different part of the sleep system, and the clinical consequences track the specific break. This preview maps the major disorders onto the architecture we have just built. Volume 2 will treat each disorder in depth.
Insomnia is the most common sleep complaint, with prevalence in clinical samples reaching 30%. The patient cannot initiate sleep, cannot maintain it, or wakes too early. Insomnia is sometimes a stand-alone disorder, often a symptom of underlying psychiatric or medical conditions, and frequently both at once. Chronic insomnia drives cardiovascular disease risk, predicts new-onset depression, impairs cognitive performance, and increases accident risk. Treatment combines cognitive-behavioral therapy for insomnia (CBT-I) — the evidence-based first-line treatment — with judicious pharmacology when needed.
Obstructive sleep apnea (OSA) fragments NREM sleep through repeated airway collapse and arousal. The patient may not be aware of the awakenings but spends little time in deep slow-wave sleep. Cardiovascular consequences are profound — OSA is an independent risk factor for hypertension, atrial fibrillation, stroke, and heart failure. Cognitive consequences include impaired memory consolidation and accelerated cognitive aging. CPAP therapy remains the most effective treatment for moderate-to-severe disease.
Narcolepsy is the loss of orexin/hypocretin neurons in the lateral hypothalamus. Orexin normally stabilizes wakefulness; its loss produces the cardinal symptoms — excessive daytime sleepiness, cataplexy (brief muscle weakness triggered by strong emotion), sleep paralysis, and hypnagogic hallucinations. REM intrudes inappropriately into waking. Treatment combines stimulants for daytime alertness with sodium oxybate or pitolisant for the underlying disorder.
REM behavior disorder (RBD), which we discussed in C12.2, reflects failure of the brainstem atonia mechanism. The patient acts out dreams. Beyond the immediate injury risk to patient and bedpartner, RBD is one of the strongest known predictors of future synucleinopathy — 80% of patients with idiopathic RBD develop Parkinson's disease, Lewy body dementia, or multiple system atrophy within 10-15 years.
Restless legs syndrome (RLS) and periodic limb movement disorder involve dopaminergic and iron-related mechanisms that produce sensory and motor symptoms disrupting sleep onset and maintenance. RLS responds to dopamine agonists, gabapentinoids, or iron repletion depending on the underlying cause.
Circadian rhythm disorders involve misalignment between the body's biological clock and the external schedule — delayed sleep phase, advanced sleep phase, shift work disorder, jet lag. These respond to timed light exposure, melatonin, and behavioral interventions targeting the suprachiasmatic nucleus.
Parasomnias — sleepwalking, night terrors, confusional arousals — typically arise from incomplete arousals out of NREM 3. They are more common in children and tend to remit with age, though stress, sleep deprivation, alcohol, and certain medications can precipitate them in adults.
The clinical takeaway: sleep complaints deserve specific evaluation, not generic reassurance. Each disorder breaks a different part of the system, has different downstream consequences, and responds to different treatments. Asking the right questions about sleep — initiation, maintenance, snoring, daytime sleepiness, restless legs, dream enactment, circadian patterns — often reveals a specific disorder underneath the generic complaint of I can't sleep.
We will return to each of these disorders in Volume 2 Stage 10. For now, hold the principle: sleep is architecture, and the disorders are specific structural failures.