Apnea is the complete cessation of breathing, defined medically as a pause in airflow lasting at least 10 seconds in adults or 20 seconds in children. A shorter pause still counts if it triggers a drop in heart rate or causes oxygen levels to fall low enough to turn the skin bluish. The term comes from the Greek “apnoia,” meaning “without breath,” and it applies to any context where breathing stops, whether during sleep, under anesthesia, or in a newborn’s first hours of life.
How Apnea Differs From Hypopnea
The distinction between apnea and hypopnea comes down to how much airflow drops. An apnea is a 90% or greater reduction in airflow. A hypopnea is a partial reduction, currently defined as a 30% to 89% drop in airflow that also causes a dip in blood oxygen or a brief awakening. Both are counted together in the apnea-hypopnea index (AHI), the primary number used to gauge severity, but a complete apnea represents a near-total or total blockage of air rather than a partial one.
The Three Types of Apnea
Obstructive Apnea
This is the most common form and happens when the physical airway collapses during sleep. Your throat muscles relax as you fall asleep, and in people with obstructive apnea, the surrounding tissue and structures press inward enough to seal off the airway. Two forces drive that collapse: the suction created by your diaphragm pulling air in, and the external pressure from tissue, fat, or enlarged tonsils and adenoids crowding the space around the airway. During waking hours, the muscles lining the throat actively hold the airway open, compensating for a naturally narrow passage. Once sleep removes that muscle tone, the airway gives way.
Central Apnea
In central apnea, the airway stays open but the brain temporarily stops sending the signal to breathe. This stems from instability in the body’s breathing control system. Normally, sensors in the brainstem and blood vessels detect rising carbon dioxide levels and trigger a breath. In central apnea, that feedback loop overreacts. After a brief pause, the system responds with overly vigorous breathing, which drives carbon dioxide too low, which then causes another pause. The cycle repeats, producing a waxing-and-waning breathing pattern. People with heart failure are particularly vulnerable because slower blood circulation delays the signal between the lungs and the brain’s sensors, amplifying the instability. When this pattern takes a distinctive crescendo-decrescendo shape, it’s called Cheyne-Stokes respiration.
Mixed Apnea
Some events begin as central apnea, with no breathing effort at all, and then transition into obstructive apnea as the airway collapses before normal breathing resumes. Ventilatory control instability can produce either central or obstructive events depending on how collapsible an individual’s airway happens to be, which is why the two types often coexist in the same person.
What Happens in Your Body During an Apnea Event
When breathing stops, oxygen levels in the blood begin to drop while carbon dioxide accumulates. The body essentially loads up on carbon dioxide during the pause and then must unload it rapidly once breathing resumes. If the breathing period between events is too short relative to the preceding pause, carbon dioxide doesn’t fully clear, and levels gradually climb over the course of the night.
Each event also triggers a stress response. The nervous system fires up, heart rate spikes, and blood pressure surges as the body fights to restart breathing. In someone with severe apnea experiencing 30 or more events per hour, this cycle of suffocation and recovery repeats hundreds of times a night, flooding the cardiovascular system with stress hormones and oxygen swings that the body was never designed to handle on a recurring basis.
Symptoms That Point to Apnea
The nighttime signs are often noticed by a bed partner rather than the person experiencing them: loud snoring, visible pauses in breathing, choking or gasping, and night sweats. The daytime picture is what typically drives someone to seek help. Excessive daytime sleepiness is the hallmark, but apnea also causes morning headaches (from elevated carbon dioxide overnight), difficulty concentrating, memory problems, heartburn, frequent nighttime urination, erectile dysfunction, and a type of insomnia where you fall asleep fine but wake repeatedly throughout the night.
Many people with apnea don’t realize they stop breathing. They may simply feel unrested despite what they thought was a full night’s sleep, or they may attribute their fatigue to aging or stress.
How Apnea Is Diagnosed and Scored
The standard diagnostic tool is a sleep study, either in a sleep lab or with a home testing device. During the study, sensors track brain waves, blood oxygen levels, heart rate, breathing patterns, chest and abdominal movement, and body position throughout the night. A technologist or algorithm then counts every apnea and hypopnea event and divides by total hours of sleep to calculate the AHI.
The American Academy of Sleep Medicine uses these AHI ranges for adults:
- Mild: 5 to fewer than 15 events per hour
- Moderate: 15 to fewer than 30 events per hour
- Severe: 30 or more events per hour
An AHI below 5 is considered normal. For context, someone with severe apnea may stop breathing for 10 to 60 seconds at a time, hundreds of times in a single night.
Children Use Different Criteria
Pediatric apnea is defined and measured differently. The 20-second threshold for children reflects their faster baseline breathing rate. More importantly, applying adult scoring rules to children misses serious cases. Research has shown that using the adult standard of counting only events lasting 10 seconds or longer fails to identify children with significant airway obstruction. In children, even one or two events per hour can be clinically meaningful, whereas adults need at least five per hour to meet the diagnostic threshold. Enlarged tonsils and adenoids are the most common cause of obstructive apnea in children, unlike in adults where excess tissue around the throat and anatomical narrowing play larger roles.
Long-Term Health Risks
Chronic, untreated apnea places sustained stress on the heart and metabolism. People with severe sleep apnea face increased risk of coronary artery disease, congestive heart failure, and stroke. Data from the Sleep Heart Health Study found that people with obstructive sleep apnea had four times the odds of developing atrial fibrillation, an irregular heart rhythm that itself raises stroke risk.
The metabolic effects are equally concerning. Repeated drops in oxygen and surges in stress hormones increase insulin resistance, the body’s reduced ability to use insulin effectively. In the same large study, abnormal blood sugar levels rose from about 9% in people with minimal apnea to 15% in those with an AHI above 15. This relationship between apnea and insulin resistance appears to hold even after accounting for obesity, suggesting that the oxygen deprivation and nervous system activation caused by apnea independently push the body toward type 2 diabetes.