The possible causes of excessive daytime sleepiness

Excessive daytime sleepiness never has a single, uniform origin. It can result from something as simple as sleep deprivation, or be the sign of a more specific disorder that deserves to be identified. Here are the most frequently encountered causes.

Specialists often classify these causes into three broad families. Sleepiness is called “induced” when it comes from an external source: sleep deprivation, shifted schedules, alcohol, medications. It's “secondary” when sleep is of poor quality because of another disorder, such as sleep apnea or restless legs. It's “primary” when the brain itself poorly regulates wakefulness, as in narcolepsy. This classification guides the doctor's approach, which always starts with the most common and easiest-to-correct causes.

The most common cause: sleep deprivation

Before considering a medical condition, the first cause to rule out is also the most mundane: insufficient sleep syndrome, meaning partial and often chronic sleep deprivation — nights that are too short, irregular bedtimes, sleep debt accumulated over several days. This is, by far, the most common cause of daytime sleepiness in the general population.

The numbers show just how widespread this sleep deprivation is. In France, the INSV's annual survey published in 2025 indicates that adults sleep a little over 7 hours a night on average. But about a quarter of them don't reach 6 hours, and sleep duration has fallen markedly over a few decades. In the United States, the CDC estimates that about one in three adults sleeps less than the recommended 7 hours. At the global level, the WHO doesn't publish a dedicated sleep statistic, but international studies describe the same trend in most industrialized countries.

Sleep deprivation is deceptive, because people appear to get used to it. Laboratory experiments have shown that volunteers limited to 6 hours of sleep per night for two weeks saw their attention performance decline as much as after one or two sleepless nights. Yet they judged themselves only “slightly” tired. A simple clue can help suspect a sleep debt: if you sleep much longer on weekends or vacation than during the week, and sleepiness clearly improves after a few days of rest, your body is probably signaling that it isn't getting enough sleep during the week.

Sleep-related breathing disorders

Sleep apnea (OSA) is among the most common medical causes of excessive daytime sleepiness. The mechanism is indirect: it isn't the lack of sleep time itself that's the problem, but its repeated fragmentation by breathing pauses, which prevents truly restorative sleep, even when its duration seems sufficient. The STOP-BANG test can help assess this possibility.

Each breathing pause ends with a few-second micro-arousal that the person retains no memory of. With 30 episodes per hour, sleep is interrupted roughly every two minutes. The deep sleep and REM sleep stages, the most restorative ones, no longer have time to set in properly. The result is an eight-hour night that's “worth” much less in terms of recovery.

Sleepiness isn't systematic, though. Some people with apnea don't feel sleepy, and conversely, a normal Epworth score doesn't rule out apnea. This is why the two questionnaires, Epworth and STOP-BANG, are complementary rather than interchangeable.

Restless legs syndrome and periodic limb movements

Restless legs syndrome causes unpleasant sensations in the legs that create an urge to move them or get up, especially in the evening or at night. It's often accompanied by periodic limb movements during sleep, which fragment sleep without the person always being aware of it. The result is daytime sleepiness despite what appears to be a normal amount of sleep.

This syndrome is more common than people think. It is thought to affect 5% to 10% of adults in Europe and North America, with a smaller share having symptoms bothersome enough to require treatment. Women are more affected, particularly during pregnancy. Known causes include iron deficiency, even without anemia: iron plays a role in dopamine function, a chemical messenger involved in controlling movement. This is why measuring ferritin, the body's iron reserve, is part of the standard workup. Certain medications, particularly antidepressants or antihistamines, can also worsen symptoms.

Circadian rhythm disorders

Shift work, night work, and highly irregular schedules desynchronize the internal biological clock from the moment the body is actually ready to sleep. The resulting sleep is often shortened or of poorer quality, with daytime sleepiness as a direct consequence.

Other rhythm disorders aren't work-related. The most common one in teenagers and young adults is delayed sleep phase syndrome. The internal clock is shifted by several hours: the person can't fall asleep before 2 or 3 a.m., but has to get up early for school or work. They then accumulate a sleep debt during the week and seem “lazy” in the morning, when in fact it's a real biological shift. Jet lag after a long trip is a temporary version of this. These disorders are managed with well-timed light exposure, regular schedules, and sometimes melatonin prescribed at specific times.

Certain medications

Many medications can cause sleepiness, whether when starting them or, sometimes, when stopping them: anti-anxiety medications, sleeping pills, certain antidepressants, antihistamines, among others. If unusual sleepiness appears after starting a new treatment, it's worth mentioning to the prescribing doctor.

In France, medications that may impair alertness carry a triangle-shaped pictogram on their packaging. The triangle is yellow for level 1 (“be careful”), orange for level 2 (“be very careful”), and red for level 3 (“do not drive”). Medications concerned also include opioid painkillers, certain anti-seizure drugs, Parkinson's disease medications, and certain blood pressure drugs. Alcohol, even in moderate amounts, potentiates the sedative effect of most of these treatments. You should never stop a treatment on your own initiative: an adjustment to the dose or timing is often enough to solve the problem.

Rarer causes worth knowing about

Some rarer conditions present mainly as severe daytime sleepiness. Narcolepsy is associated with a genetic predisposition, and its origin remains partly poorly understood. Idiopathic hypersomnia is characterized by near-constant sleepiness for more than three months, with naps that are only slightly or not at all refreshing. Mood disorders such as depression can also come with excessive daytime sleepiness, present in 10% to 40% of affected people according to studies.

Narcolepsy affects about 20 to 50 people per 100,000. In its most typical form, called type 1, it is explained by the loss of a few tens of thousands of neurons that produce hypocretin, a chemical messenger that maintains wakefulness. An autoimmune mechanism is currently the leading hypothesis. Diagnosis is often delayed, sometimes by several years, because the first symptoms often appear in adolescence and are mistaken for laziness or a lack of motivation.

Other medical conditions can also cause sleepiness: underactive thyroid, kidney or liver failure, head trauma, certain neurological diseases such as Parkinson's disease. For depression, the link runs both ways: lack of sleep undermines mood, and a depressed mood disrupts sleep. The hub's Am I depressed? test can help explore this possibility.

Why identifying the precise cause matters

The approach to treatment differs radically depending on the cause: sleep hygiene for sleep deprivation, specific treatment for sleep apnea or restless legs syndrome, treatment adjustment for a medication-related cause. First quantifying the level of sleepiness with a tool such as the Epworth scale is therefore a useful first step before discussing it with a healthcare professional, who can then direct you toward the appropriate further testing if needed.

To prepare for this consultation, a sleep diary kept for two weeks is often valuable. Simply note down, each day, bedtime, wake time, nighttime awakenings, naps, coffee and alcohol intake, and moments of sleepiness. This simple document lets the doctor spot a sleep debt, a shifted schedule, or a link to a medication within a few minutes. Several causes can, in fact, coexist: someone can both sleep too little and have sleep apnea, and correcting only one of the two problems won't be enough.