Hypersomnolence and circadian rhythm sleep-wake disorders

Disorder

Being very sleepy despite enough sleep, or sleeping well but at the wrong times because the body clock is out of step with life.

Updated Nov 13, 2026

Key points

Idiopathic hypersomnia differs from narcolepsy by the lack of cataplexy, sleep paralysis and hypnagogic hallucinations.
In idiopathic hypersomnia, the multiple sleep latency test shows short sleep latency (under 8 minutes) without REM periods, and polysomnography shows no other sleep abnormality.
Delayed sleep phase is typical of adolescents (late to sleep, late to wake); advanced phase is typical of older adults (early to sleep, early to wake).
Jet lag follows travel across more than two time zones and is worse flying east; non-24-hour rhythm is more common in blind people.
Light is a strong signal for the clock: a light box on waking shifts sleep earlier and light in the late afternoon or early evening shifts it later; for delayed phase, melatonin is taken 4 to 5 hours before the desired bedtime.
Before diagnosing idiopathic hypersomnia, polysomnography must show no other sleep disorder, and other medical conditions are ruled out.

What it is

Idiopathic hypersomnia is excessive daytime sleepiness with difficulty waking and sleep inertia, with or without a long night's sleep (more than 10 hours), when polysomnography shows no other sleep disorder. Circadian rhythm sleep-wake disorders arise when the internal clock is out of line with the schedule a person needs, causing insomnia, sleepiness or both; people have no symptoms when they can follow their natural cycle. Delayed phase is more common in adolescents and advanced phase in older adults.

Key facts

Sleep inertia, a period of drowsiness and difficulty waking, is typical of idiopathic hypersomnia.
In delayed sleep phase, sleep is normal when the person can follow the natural cycle, for example by sleeping late on holidays.
Shift work symptoms grow with more frequent counterclockwise (sleep-advancing) changes; fixed shifts are biologically preferable, and rotations should go clockwise, from day to evening to night.
A sleep diary, and actigraphy with a motion sensor worn for 3 to 14 days, show the timing and regularity of sleep.

Too sleepy, or asleep at the wrong time

Disorder	Typical patient	Pattern	Treatment principle
Idiopathic hypersomnia (hypersomnolence disorder)	Sleepy despite enough sleep	Long night sleep in some, difficulty waking, sleep inertia	Wake-promoting medicines
Narcolepsy	Excessive daytime sleepiness	Cataplexy, sleep paralysis or hypnagogic hallucinations	Wake-promoting medicines
Delayed sleep phase	Adolescent	Cannot fall asleep until very late, cannot wake for school or work	Light on waking, melatonin 4 to 5 hours before the desired bedtime, regular wake time
Advanced sleep phase	Older adult	Sleepy in the early evening, wakes very early	Bright light in the evening, light-blocking goggles in the morning
Jet lag	Traveller crossing more than two time zones	Insomnia and daytime sleepiness, worse flying east	Light therapy timed to the direction of travel
Shift work type	Night or rotating shift worker	Insomnia by day and sleepiness at work	Nap before the shift, light-blocking glasses outdoors by day, dark quiet bedroom
Non-24-hour rhythm	Blind person	Sleep times drift later day after day	Melatonin receptor agonists

Clinical notes

Start with a careful history, a sleep diary and a review of drugs, mood and medical illness; refer for polysomnography and a multiple sleep latency test when idiopathic hypersomnia or narcolepsy is suspected.
Idiopathic hypersomnia is treated with wake-promoting medicines such as modafinil or oxybates.
For circadian disorders, timing matters: a light box just after waking shifts the clock earlier, while light in the late afternoon or early evening shifts it later.
Keep regular sleep and waking times, and avoid caffeine within eight hours of the desired bedtime.
Shift workers may benefit from a short nap before the shift, light-blocking glasses when outside during the day, and a dark, quiet place to sleep.

Test yourself

Think of the answer, then tap the question to see it.

Which clinical features separate idiopathic hypersomnia from narcolepsy?

Idiopathic hypersomnia has no cataplexy, sleep paralysis or hypnagogic hallucinations, and the sleep latency test shows short latency without REM periods.

A 16-year-old cannot fall asleep before 3 a.m. and cannot wake for school, but sleeps normally on holidays. What is the likely diagnosis?

Delayed sleep phase type of circadian rhythm sleep-wake disorder.

Which light exposure helps advanced sleep phase in an older adult?

Bright light in the evening, with light-blocking goggles in the morning.

Why is jet lag usually worse after flying east?

Eastward travel requires advancing the sleep cycle, which causes more severe symptoms than delaying it.

In whom is non-24-hour sleep-wake rhythm most common?

In blind people, in whom it is more common.

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Educational content on behavior and the clinical relationship; not a diagnostic or treatment guide and no substitute for assessment by a health professional. If you or someone near you is at risk, seek help now from an emergency service or a mental-health helpline.

Sources: Idiopathic Hypersomnia (MSD Manual Professional Edition, 2026); Circadian Rhythm Sleep Disorders (MSD Manual Professional Edition, 2026); Circadian Rhythm Disorders: Diagnosis (National Heart, Lung, and Blood Institute (NHLBI), 2026); Circadian Rhythm Disorders: Treatment (National Heart, Lung, and Blood Institute (NHLBI), 2026).

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