During sleep apnea, your breathing repeatedly stops and restarts throughout the night, triggering a cascade of stress responses that affect your heart, blood oxygen levels, metabolism, and brain. Each pause can last from a few seconds to over a minute, and in severe cases, this cycle repeats 30 or more times per hour. What feels like poor sleep on the surface is actually your body fighting for air hundreds of times a night.
What Happens During Each Breathing Pause
In obstructive sleep apnea, the most common form, the muscles in your throat relax too much during sleep and physically collapse inward, blocking your airway. Air can’t get through even though your chest and diaphragm are still trying to pull it in. This creates a sharp drop in pressure inside your chest, and your blood oxygen levels begin to fall. Normal oxygen saturation sits between 95% and 100%. During an apnea event, that can plummet to 70% or even 60%.
As oxygen drops, your body’s fight-or-flight system kicks in. Your nervous system fires a burst of adrenaline-like activity, spiking your heart rate and blood pressure to force you awake just enough to reopen the airway. You gasp, take a few deep breaths, oxygen levels recover, and you drift back to sleep, often without ever becoming fully conscious. Then the whole cycle starts again.
Central sleep apnea works differently. The airway stays open, but the brainstem, which controls automatic breathing, temporarily stops sending signals to the breathing muscles. Your body simply doesn’t try to breathe. This form is less common and is often linked to conditions that affect the brainstem, such as heart failure or stroke.
How Severity Is Measured
Doctors measure sleep apnea using the apnea-hypopnea index, or AHI, which counts the number of breathing interruptions per hour of sleep. A full pause is an apnea; a partial blockage that reduces airflow is a hypopnea. The scale, established by Harvard’s sleep medicine program, breaks down like this:
- Mild: 5 to 14 events per hour
- Moderate: 15 to 29 events per hour
- Severe: 30 or more events per hour
Someone with severe sleep apnea may experience over 200 breathing interruptions in a single night. Each one triggers an arousal from sleep, even if it’s too brief to remember. This is why people with sleep apnea can spend eight or nine hours in bed and still wake up exhausted. The architecture of sleep, particularly the deep and REM stages, gets shattered into fragments.
The Cardiovascular Toll
The repeated surges of stress hormones and blood pressure spikes don’t just disrupt sleep. They reshape your cardiovascular system over time. Each apnea event trains your nervous system to stay in a heightened state, and eventually that elevated baseline carries over into waking hours. People with untreated obstructive sleep apnea develop sustained high blood pressure, not just during the night but around the clock.
The American Heart Association identifies obstructive sleep apnea as an independent risk factor for a range of serious cardiovascular problems: atrial fibrillation, heart failure, coronary artery disease, and stroke. The association with coronary events is particularly stark. Untreated sleep apnea roughly doubles the risk of cardiovascular events or death. It also increases the likelihood of stroke recurrence in people who have already had one, and worsens both survival and cognitive recovery afterward.
The mechanism is straightforward. Repeated oxygen drops and pressure swings damage blood vessel walls, promote inflammation, and accelerate the buildup of arterial plaque. Sleep apnea has been linked to plaque instability, meaning existing buildup in the arteries is more likely to rupture and cause a heart attack or stroke.
Metabolic and Hormonal Effects
The oxygen roller coaster of sleep apnea doesn’t just stress the heart. It disrupts how your body handles blood sugar. Intermittent drops in oxygen trigger inflammation and oxidative stress throughout the body, which interfere with insulin’s ability to move sugar out of the bloodstream. The liver’s ability to clear insulin is also impaired. Over time, this pushes the body toward insulin resistance, the precursor to type 2 diabetes.
Sleep fragmentation compounds the problem. Poor sleep independently raises cortisol (a stress hormone), lowers levels of hormones that regulate appetite, and shifts metabolism toward fat storage. People with untreated sleep apnea often find it unusually difficult to lose weight, and the excess weight in turn worsens the airway obstruction, creating a cycle that’s hard to break without treatment.
What Treatment Looks Like
The most common treatment is continuous positive airway pressure, or CPAP. A small machine delivers a steady stream of pressurized air through a mask you wear while sleeping, holding the airway open so it can’t collapse. Most people need a pressure setting between 8 and 10 cmH2O, though machines can range from 4 to 20. Auto-adjusting versions monitor your breathing in real time and increase or decrease the pressure as needed throughout the night, typically within a preset range like 5 to 20 cmH2O.
CPAP is effective when used consistently, but many people struggle with the mask, the noise, or the sensation of pressurized air. Comfort improvements in newer machines have helped, but adherence remains a challenge.
For people who can’t tolerate CPAP, a surgically implanted nerve stimulator is an alternative. The device activates the nerve that controls tongue movement, gently pushing the tongue forward during sleep to keep the airway clear. Eligibility is typically limited to people with moderate to severe obstructive sleep apnea and a BMI of 35 or below. Clinical data shows that the benefits of nerve stimulation hold steady through at least three years, with improvements in both breathing metrics and quality of life comparable to CPAP.
Other options include custom oral appliances that reposition the jaw, positional therapy for people whose apnea worsens when sleeping on their back, and in some cases surgical procedures to remove excess tissue from the throat. Weight loss, when applicable, can significantly reduce the severity of obstructive sleep apnea and sometimes resolve it entirely.
Daytime Symptoms Most People Notice First
Because the breathing pauses happen during sleep, many people with sleep apnea don’t know they have it. The symptoms that eventually bring them to a doctor are daytime ones: persistent fatigue despite a full night in bed, morning headaches caused by overnight drops in oxygen, difficulty concentrating, irritability, and a feeling of mental fog that doesn’t improve with caffeine or naps. Bed partners often notice the problem before the person with apnea does, hearing loud snoring punctuated by silent pauses and sudden gasps.
Untreated, these daytime effects go beyond discomfort. The level of sleepiness associated with moderate to severe sleep apnea significantly increases the risk of car accidents and workplace injuries. Cognitive function, including memory and reaction time, deteriorates in ways that mimic chronic sleep deprivation, because that’s essentially what it is: the brain never completes the restorative cycles it needs.