Daytime sleepiness in a nutshell

What is sleepiness?

Daytime sleepiness refers to an involuntary tendency to fall asleep, or to fight the urge to sleep, outside of one's usual sleeping hours. It isn't simply a feeling of weariness. It's a genuine physiological signal that pushes the body toward falling asleep, sometimes in situations where staying awake matters: while driving, in a meeting, in the middle of a conversation.

Sleepiness plays out along a continuum. At one end, there is a simple drop in alertness: rereading the same line three times, losing track of a movie. Next come physical signs, such as yawning, heavy eyelids, or the head nodding. At the far end are involuntary episodes of falling asleep, sometimes lasting only a few seconds. These “microsleeps” often go unnoticed by the person themselves, who has the impression of having stayed awake when their brain briefly switched into sleep.

Sleepiness is as common as it is overlooked. In France, a survey by the Institut national du sommeil et de la vigilance (INSV) conducted among adults aged 18 to 60 found an Epworth score above 10 — the usual threshold for excessive sleepiness — in about one adult in five (19%). About 3% had severe sleepiness. Only a minority of these people had mentioned it to a doctor. In the United States, a survey by the Centers for Disease Control and Prevention (CDC) showed that nearly 38% of adults had fallen asleep unintentionally during the day at least once in the previous month. At the global level, the World Health Organization (WHO) doesn't publish a dedicated figure. Studies conducted in Europe, North America, and Asia generally put excessive daytime sleepiness at between 10% and 20% of adults, depending on the definitions used.

A brief physiological explanation

Sleep, and therefore the sleepiness that precedes it, is regulated by two mechanisms acting in parallel. The first, the homeostatic process, works like an hourglass. A substance called adenosine builds up in the brain throughout wakefulness, and the more it builds up, the greater the pressure to sleep becomes. It then drops back down during the sleep that follows. The second, the circadian process, is driven by an internal biological clock, the suprachiasmatic nucleus, located in the brain. It determines when the body is naturally ready to sleep, independent of how long you've been awake.

It's the interaction between these two processes that explains some familiar experiences: a wave of tiredness in the early afternoon despite a good night's sleep, or conversely, a feeling of being surprisingly alert late in the evening after a demanding day.

This model, described by Swiss researcher Alexander Borbély in the early 1980s, also sheds light on everyday habits. Caffeine works precisely by blocking adenosine receptors. It therefore temporarily masks the pressure to sleep, but doesn't make it disappear, which explains the “rebound” once its effect wears off. The circadian clock, meanwhile, resets each day mainly through morning light, which suppresses the secretion of melatonin, the hormone of night. Bright screen light in the evening works in the opposite direction: it delays the sleep signal and can shorten the night.

The afternoon dip, roughly between 1 p.m. and 4 p.m., isn't therefore just about the meal. It corresponds to a small drop in alertness programmed by the internal clock, present even in people who skip lunch. A heavy or alcohol-accompanied lunch only makes it more pronounced.

Sleepiness and fatigue: two different things

This is a common source of confusion, including in medical consultations: fatigue and sleepiness are not the same thing. Fatigue is a feeling of physical or mental exhaustion, which generally improves with rest, not necessarily with sleep. Sleepiness is an involuntary tendency to fall asleep, particularly in calm or understimulating situations: reading, watching television, riding in a car.

It's entirely possible to be tired without being sleepy, and conversely, to be sleepy without feeling particularly tired. This distinction matters, because the causes and the approach to addressing them differ depending on which one you're actually dealing with.

Two examples illustrate this difference. Someone going through a depressive episode or burnout may feel exhausted all day and yet have great difficulty falling asleep at night: they are tired, not sleepy. Conversely, someone with undiagnosed sleep apnea may feel “fine” and still fall asleep in front of the television every evening, or as soon as they're a passenger in a car. A simple test helps tell the difference: if you lie down in a quiet room in the early afternoon, do you fall asleep within a few minutes, or do you stay awake while feeling drained? In the first case, sleepiness dominates. In the second, it's more likely fatigue.

When should you worry?

Occasional sleepiness, after a short night or an unusual period, is nothing to worry about. However, certain signs should raise concern:

• Sleepiness that persists despite enough time asleep
• Episodes of dozing off in situations where staying awake matters (while driving, at work)
• Sleepiness that settles in over time, over several weeks, with no obvious cause

In these cases, sleepiness may be a sign of an underlying disorder. Sleep apnea is a prominent one among the possible causes, but there are others: circadian rhythm disorders, the effects of certain medications, narcolepsy.

Other, rarer signs deserve a prompt conversation with a doctor when they accompany sleepiness. The first is sudden, irresistible sleep onset in the middle of an activity, for example while eating or talking. The second is a sudden loss of muscle tone triggered by an emotion, such as a fit of laughter, where the knees buckle or the head drops. The third is a sensation of paralysis when falling asleep or waking up, accompanied by very vivid hallucinations. These symptoms are suggestive of narcolepsy, a rare condition but one that warrants a specialist opinion.

Finally, sleepiness that appears in someone who had until then been sleeping very well, with no change in lifestyle or medication, always deserves to be mentioned.

Measuring your sleepiness objectively

Sleepiness remains, above all, a subjective feeling. It's therefore useful to have a structured tool to assess it more precisely than a simple impression would. That's exactly what the Epworth Sleepiness Scale offers: a validated questionnaire used worldwide, which quantifies the level of sleepiness felt in several everyday situations.

Developed in 1991 by Australian physician Murray Johns, at Melbourne's Epworth Hospital, from which it takes its name, the scale asks you to estimate the chance of dozing off in eight situations. These include reading, watching television, riding as a passenger, having a conversation, or being stopped at a red light while driving. Each situation is scored from 0 (no chance) to 3 (high chance), for a total of 0 to 24. A score up to 10 is considered normal. Beyond that, it's referred to as excessive daytime sleepiness, more pronounced the higher the score. Its main value is turning a vague impression — “I'm a bit tired” — into a number that can be compared over time.

The scale also has its limits. It measures what the person perceives, and many people underestimate their own sleepiness, especially when it has set in gradually. It can be illuminating to ask someone close to you whether they see you dozing off more often than you think. When doubt persists, sleep centers have objective tests performed in a lab. The multiple sleep latency test measures how quickly someone falls asleep during several naps offered throughout the day. The maintenance of wakefulness test measures the ability to stay awake in a quiet, dark room.

The Epworth score isn't a diagnosis. It's a starting point for a conversation with a healthcare professional. It's often used alongside the STOP-BANG score, which assesses the risk of sleep apnea. If your result is above 10, or if you've already dozed off while driving, it's worth mentioning to your doctor.