What is sleep apnea?

Obstructive sleep apnea-hypopnea syndrome (OSAHS), better known as sleep apnea, is characterized by abnormally frequent pauses in breathing — apneas — or reductions in airflow — hypopneas — during sleep. These episodes occur when the upper airway partially or completely collapses, preventing air from flowing normally. They can repeat dozens of times per hour without the person retaining the slightest memory of it on waking.

By convention, an apnea is defined as an interruption in airflow lasting at least 10 seconds. A hypopnea is a clear drop in that airflow, over the same minimum duration, accompanied by a drop in blood oxygen or a brief brain arousal. Many episodes actually last 20 to 30 seconds, sometimes longer. A distinction also needs to be made between obstructive apnea, by far the most common form, and central apnea, which is rarer. In the obstructive form, the brain correctly commands breathing and the chest works to inhale, but the air runs into a closed pharynx. In the central form, it is the breathing command itself that briefly stops. The STOP-BANG score and this article focus on the obstructive form.

A massively widespread condition, largely underdiagnosed

Globally, a landmark study published in 2019 in The Lancet Respiratory Medicine (Benjafield et al.) estimates that around 936 million adults aged 30 to 69 have at least mild sleep apnea. Of these, about 425 million are estimated to have a moderate-to-severe form. This figure is nearly ten times higher than the WHO's previous estimate, which dated from 2007 and put the number at around a hundred million people. This gap is mainly due to changes in measurement methods and diagnostic criteria, not a sudden surge in the condition itself. According to the authors of this work, more than 85% of affected people would remain undiagnosed.

In the United States, the American Academy of Sleep Medicine (AASM) estimates that about 30 million adults, or 12% of the adult population, have obstructive sleep apnea. About 80% of them would remain undiagnosed.

In France, official estimates put OSAHS at between 2% and 5% of the adult population, or roughly 1 to 3 million people. The number of patients treated with continuous positive airway pressure has risen sharply and now exceeds 1.5 million, but it remains lower than the number of people likely affected.

These figures can seem contradictory: the Lancet study, for instance, attributes more than 20 million cases to France. The difference comes down to what is being counted. The global study counts anyone whose sleep recording shows at least 5 episodes per hour, whether or not they have symptoms. The French figures count the syndrome — that is, apneas associated with symptoms, such as sleepiness or fatigue. The American Wisconsin cohort, published in 1993, already illustrated this gap. An index of at least 5 episodes per hour was found in 24% of men and 9% of women of middle age. The symptomatic syndrome affected only 4% of men and 2% of women. Both measurements are accurate: they simply answer different questions.

A condition that rises markedly with age

The frequency of sleep apnea rises markedly with age. Beyond age 60, a significant proportion of the population shows at least a mild form on a sleep recording. The Swiss HypnoLaus study (Heinzer et al., 2015), conducted among adults aged 40 to 85 using modern recording methods, found at least moderate apnea in nearly one man in two and nearly one woman in four. These figures are a reminder that some of the apnea detected in older people remains mildly symptomatic. Treatment is always discussed on a case-by-case basis.

Men are affected roughly twice as often as women. This gap narrows after menopause, when the frequency rises noticeably in women. Sex hormones, particularly progesterone, appear to be partly protective: they stimulate breathing and influence the tone of the muscles that keep the pharynx open. The shift toward more abdominal and neck fat after menopause probably also plays a role.

Why the airway collapses

The pharynx, the back of the throat, is a flexible tube with no bone or cartilage support along much of its length. During wakefulness, dilator muscles — including the genioglossus, which pulls the tongue forward — keep it continuously open. During sleep, their activity naturally decreases, especially during REM sleep, the dreaming phase. For most people, this causes no problem. If the pharynx is already narrow or particularly “floppy,” it closes.

A cycle then kicks in and repeats all night long. The pharynx closes, the chest strains against the obstruction, and blood oxygen drops. The brain eventually triggers a few-second micro-arousal, which restores muscle tone. Breathing resumes, often with a loud snore or a gasp. The person falls back asleep immediately, without perceiving anything, and the cycle starts again.

Several factors promote this obstruction. The first is the natural loosening of tissue with age. Being overweight, and especially obesity, plays a major role, through fat infiltration around the pharynx and at the base of the tongue, but also through the drop in lung volume when lying down, which “pulls” less on the airway — hence the relevance of neck circumference in the STOP-BANG score. Certain anatomical features also contribute: a narrow or recessed jaw, enlarged tonsils, a large tongue, nasal obstruction. Finally, alcohol and sedatives relax muscle tone and delay the protective micro-arousal.

Research shows that anatomy doesn't explain everything. In some patients, the problem also stems from an insufficient muscular response, a too-low arousal threshold, or unstable breathing control. This is why two people of similar build can have very different profiles, and why slim people can be affected too.

Symptoms to know

Signs of sleep apnea fall into two groups:

At night: often loud snoring, breathing pauses noticed by a bed partner, sensations of choking, restless and fragmented sleep, frequent need to urinate, excessive sweating.

During the day: fatigue on waking despite a full night's sleep, daytime sleepiness, morning headaches, difficulty concentrating. These symptoms are easily attributed to other causes, such as stress or overwork, as long as the link with sleep quality hasn't been made.

The need to urinate at night (nocturia) often comes as a surprise, but it has a clear explanation. Breathing efforts against a closed pharynx create strong pressure swings in the chest, which stretch the chambers of the heart. The heart then releases a hormone that increases urine production. Morning headaches, meanwhile, are thought to be linked to carbon dioxide buildup and poor sleep quality. Taken individually, these signs have many other possible causes. It's their combination that points the way.

A point that's often overlooked: in women, symptoms are frequently less typical. Snoring is quieter, but persistent fatigue, insomnia, headaches, and mood changes are more common. As a result, diagnosis is often made later than in men — a delay also seen for other conditions whose presentation differs by sex, such as heart attacks. For a woman seeking care for unexplained fatigue or insomnia, it can be worth considering this possibility, especially after menopause.

Why it matters to address this early

Sleep apnea remains largely underdiagnosed: many affected people are unaware of it, even though solutions exist once a diagnosis is made. This delay has several causes. The person is asleep during their apneas and has no awareness of them. Daytime symptoms set in gradually and end up seeming “normal.” Finally, snoring and fatigue are socially normalized. In the United States, public health data show that only about a third of adults with suggestive symptoms had mentioned them to a healthcare professional.

Spotting the warning signs, for example with the STOP-BANG score, is often the first step before considering a dedicated consultation. If your main symptom is a tendency to fall asleep during the day, the Epworth Sleepiness Scale, also available on the hub, measures this specific dimension. Neither of these questionnaires makes a diagnosis. Their role is to help you decide whether it's worth talking to your doctor, in a context where the vast majority of affected people worldwide are still undiagnosed.