The Most Common Sleep Disorders

There are over 80 recognised sleep disorders, but a handful account for the vast majority of clinical presentations. Understanding what these common conditions are, how they differ from one another, and what treatment looks like for each is an important step — both for those seeking answers for themselves and for anyone supporting a loved one through sleep difficulties.

Insomnia Disorder

Insomnia is the most common sleep disorder in the world, affecting an estimated 30% of adults in some form and approximately 10% with the full clinical picture of chronic insomnia disorder.

Insomnia is defined by persistent difficulty falling asleep, staying asleep, or waking too early — occurring at least three nights per week for at least three months — that causes significant daytime impairment (fatigue, concentration problems, mood changes, reduced performance). Crucially, insomnia occurs despite adequate opportunity and a suitable environment for sleep — it is not simply a consequence of being too busy to sleep.

Insomnia has a bidirectional relationship with anxiety, depression, and stress — it both causes and is caused by these conditions, creating self-reinforcing cycles that often require specific treatment to break.

Gold standard treatment: Cognitive Behavioral Therapy for Insomnia (CBT-I) — a structured programme addressing the thoughts and behaviours maintaining insomnia. More effective than sleep medications in the long term and without the risks of dependence.

Obstructive Sleep Apnea (OSA)

The most common medical sleep disorder, obstructive sleep apnea affects an estimated 936 million adults globally — yet the majority remain undiagnosed. In OSA, the muscles of the upper airway relax during sleep, causing the airway to collapse and temporarily stop breathing. Each collapse (an apnea) triggers a brief arousal — often too brief to be consciously remembered — to restart breathing. This can happen hundreds of times per night.

Key presenting symptoms include loud snoring, witnessed breathing pauses or gasping episodes, morning headaches, excessive daytime sleepiness (often mistaken for laziness or depression), and difficulty concentrating. Many people with significant OSA do not report feeling unwell — their baseline has shifted so gradually that profound fatigue feels normal.

OSA is a serious medical condition associated with hypertension, heart disease, stroke, atrial fibrillation, type 2 diabetes, and elevated accident risk. Diagnosis requires a sleep study (home or in-lab).

Gold standard treatment: CPAP (Continuous Positive Airway Pressure) — a bedside device delivering pressurised air through a mask to keep the airway open throughout the night. Transformative when used consistently. Mandibular advancement devices and weight loss are alternative or complementary approaches for appropriate patients.

Restless Legs Syndrome (RLS)

Also called Willis-Ekbom disease, restless legs syndrome is a neurological sensorimotor disorder affecting 5–10% of adults. The core symptom is an uncomfortable, often irresistible urge to move the legs — typically accompanied by sensations described as crawling, creeping, pulling, or aching — that occurs or worsens at rest and is relieved by movement. Symptoms follow a clear circadian pattern, being worst in the evening and at night.

RLS is frequently dismissed or misattributed to anxiety, growing pains, or poor circulation. It is a genuine neurological condition with established pathophysiology involving dopamine dysregulation and iron metabolism in the brain.

Secondary RLS is particularly common in iron deficiency (even without anaemia), pregnancy, chronic kidney disease, and as a medication side effect (antidepressants, antihistamines, antipsychotics). Checking serum ferritin is the most important first investigation.

First-line treatment: Iron supplementation where deficiency is identified; gabapentinoids (pregabalin, gabapentin); dopamine agonists (with caution due to augmentation risk); lifestyle modifications.

Narcolepsy

Narcolepsy is a chronic neurological disorder of sleep-wake regulation affecting approximately 1 in 2,000 people. The hallmark is excessive daytime sleepiness of a severity and quality distinct from ordinary tiredness — including irresistible sleep attacks that can occur at any time. In narcolepsy type 1, this is accompanied by cataplexy (sudden loss of muscle tone triggered by strong positive emotions), sleep paralysis, and hypnagogic/hypnopompic hallucinations.

The cause of narcolepsy type 1 is the autoimmune destruction of orexin-producing neurons in the hypothalamus. The average time from symptom onset to diagnosis is still 10–15 years in many countries — a reflection of how poorly understood and underrecognised the condition remains.

Treatment: Wakefulness-promoting agents (modafinil, armodafinil) for daytime sleepiness; sodium oxybate or pitolisant for both EDS and cataplexy; strategic scheduled napping; SNRIs for cataplexy.

Circadian Rhythm Sleep-Wake Disorders

This group of conditions arises when an individual's internal biological clock — the circadian rhythm — is misaligned with their external environment or social obligations. The most common forms include:

• Delayed Sleep-Wake Phase Disorder (DSWPD): The internal clock is shifted late — falling asleep and waking much later than socially expected. Common in adolescents and young adults. Often dismissed as a lifestyle choice, but it is a genuine neurobiological condition.
• Shift Work Disorder: Chronic misalignment in workers who work outside conventional day hours. Associated with insomnia during desired sleep time and excessive sleepiness during work.
• Jet Lag Disorder: Transient misalignment following rapid transmeridian travel across multiple time zones.

Treatment: Timed bright light therapy and timed low-dose melatonin are the most effective interventions for phase-shifting the circadian clock. Consistency of schedule is essential.

Parasomnias

Parasomnias are a diverse group of disorders involving abnormal behaviours, movements, emotions, or perceptions arising from sleep or sleep-wake transitions. The most clinically important include:

• NREM parasomnias (disorders of arousal): Sleepwalking, sleep terrors, and confusional arousals — arising from deep NREM sleep, most commonly in children, often resolving in adolescence
• REM Sleep Behaviour Disorder (RBD): Physically acting out vivid dreams — a clinically serious condition associated with subsequent neurodegenerative disease (Parkinson's, Lewy body dementia) in up to 90% of cases
• Nightmare Disorder: Recurrent distressing dreams causing significant daytime impairment — particularly common in PTSD

Periodic Limb Movement Disorder (PLMD)

PLMD involves repetitive, involuntary jerking movements of the legs (and sometimes arms) during sleep — typically occurring every 20–40 seconds and often fragmenting sleep without the person being aware. Closely associated with RLS (approximately 80% of people with RLS also have PLMD). Diagnosed by polysomnography showing leg EMG activity during sleep. Treatment parallels that of RLS when clinically significant.

Can You Have More Than One Sleep Disorder?

Yes, and comorbid sleep disorders are common. The most frequently seen combinations include:

• OSA plus insomnia (called COMISA) — one of the most treatment-resistant combinations, affecting an estimated 30–40% of those with either condition
• RLS plus PLMD — they frequently co-occur and share pathophysiology
• Narcolepsy plus sleep apnea — both cause EDS by different mechanisms
• Any sleep disorder plus insomnia arising from anxiety about sleep

Comorbid sleep disorders require both conditions to be identified and addressed for treatment to be fully effective. This is one reason comprehensive sleep evaluation is more productive than targeted single-condition testing when clinical presentation is complex.

References

• American Academy of Sleep Medicine. International Classification of Sleep Disorders, 3rd edition. 2014.
• Morin CM, Benca R. Chronic insomnia. The Lancet. 2012;379(9821):1129–1141.
• Benjafield AV, et al. Estimation of the global prevalence of sleep apnea. The Lancet Respiratory Medicine. 2019;7(8):687–698.
• National Institute of Neurological Disorders and Stroke. Sleep Disorders Overview.