A sleep study records your brain waves, breathing, heart rhythm, oxygen levels, and body movements throughout the night, then translates all of that data into a detailed picture of what’s happening while you sleep. The formal name is polysomnography, and it remains the gold standard for diagnosing conditions like sleep apnea, narcolepsy, restless legs, and parasomnias such as sleepwalking. Here’s what each piece of that data actually tells you and your doctor.
Brain Waves and Sleep Stages
Small sensors placed on your scalp record electrical activity in your brain, which is how technicians determine exactly which sleep stage you’re in at any given moment. A normal night cycles through four stages in a predictable pattern. N1 is light sleep and accounts for roughly 5% of the night. N2 is a slightly deeper stage that makes up about 45%. N3 is the deepest, most restorative phase at around 25%. REM sleep, the stage most associated with vivid dreaming, fills the remaining 25%.
Your final report breaks down how much time you spent in each stage, how long it took you to fall asleep initially, and how many times you woke up. If your deep sleep percentage is unusually low, that can explain why you feel exhausted despite spending eight hours in bed. If you’re barely reaching REM, it points toward a different set of problems. This breakdown of your night, called sleep architecture, is one of the most valuable parts of the report because no other test can measure it.
Breathing and Sleep Apnea Severity
Sensors track airflow at your nose and mouth while elastic belts around your chest and abdomen measure how hard your body is working to breathe. Together, these detect two types of events: apneas (complete pauses in breathing) and hypopneas (partial reductions in airflow). The number you’ll see on your report is the apnea-hypopnea index, or AHI, which is the average number of these events per hour of sleep.
An AHI below 5 is considered normal. Between 5 and 15 is mild sleep apnea. Between 15 and 30 is moderate. Above 30 is severe. Someone with severe apnea is experiencing more than 30 breathing disruptions every hour, often without being aware of it. The study also distinguishes between obstructive apnea, where your airway physically collapses, and central apnea, where your brain temporarily stops sending the signal to breathe. That distinction matters because the treatments are different.
Blood Oxygen Levels
A small clip on your finger continuously measures the oxygen saturation in your blood. Every time your breathing pauses or slows, oxygen levels can dip. The report captures this as the oxygen desaturation index: how many times per hour your blood oxygen drops by 3% or more from your baseline. It also records your lowest oxygen reading of the night.
This matters because repeated oxygen drops are what cause much of the cardiovascular strain linked to untreated sleep apnea. Two people can have a similar AHI but very different oxygen profiles, so this measurement adds important context to the breathing data alone.
Heart Rhythm
A single-lead heart monitor runs throughout the study. It’s not as comprehensive as a full cardiac workup, but it picks up abnormal rhythms that occur during sleep. Technicians look for episodes of rapid heart rate (both narrow and wide complex tachycardia), slow heart rate, and irregular rhythms. Sleep apnea in particular can trigger heart rhythm disturbances during the oxygen drops that follow each breathing pause, so finding these patterns together strengthens the diagnosis and can flag cardiac risks you didn’t know about.
Leg and Body Movements
Sensors on your shins track leg movements throughout the night. Periodic limb movements, repetitive twitching or jerking of the legs during sleep, are surprisingly common and can fragment sleep without you ever fully waking. A periodic limb movement index above 15 per hour is considered abnormal in adults. In children, the threshold is lower, at 5 per hour.
The study also captures other body movements, position changes, and whether you spent most of the night on your back, your side, or shifting between the two. Some people only stop breathing when they sleep on their back, a pattern called positional apnea that opens up simpler treatment options.
Eye Movements and Muscle Tone
Sensors near your eyes record eye movement patterns, which are essential for identifying REM sleep. During normal REM, your body enters a state of temporary paralysis so you don’t physically act out your dreams. A chin sensor measures this muscle tone. If the study shows you’re moving, talking, or thrashing during REM, that points toward REM sleep behavior disorder, a condition linked to neurodegenerative diseases like Parkinson’s. Catching it early can be significant.
What the Daytime Follow-Up Test Shows
If narcolepsy is suspected, you may stay the next day for a Multiple Sleep Latency Test. You’re given five 20-minute nap opportunities spaced two hours apart while still wearing the sensors. The test measures two things: how quickly you fall asleep on average and whether you enter REM sleep abnormally fast during those naps.
Falling asleep in 8 minutes or less on average, combined with entering REM during two or more of the five naps, is diagnostic for narcolepsy in the right clinical context. Healthy sleepers rarely drop into REM during a short daytime nap, so this pattern is a strong signal.
Home Tests vs. In-Lab Studies
Home sleep apnea tests are portable, more convenient, and work well for straightforward cases of suspected obstructive sleep apnea. They typically measure airflow, breathing effort, and oxygen levels. What they don’t measure is brain activity, which means they can’t determine your sleep stages, detect REM behavior disorder, identify narcolepsy, or accurately measure periodic limb movements. They also can’t tell when you’re actually asleep versus lying awake, which can make breathing scores less precise.
For uncomplicated sleep apnea in adults, a home test is often enough to confirm the diagnosis and start treatment. But if your symptoms are more complex, if initial results are inconclusive, or if conditions like narcolepsy or parasomnias are on the table, an in-lab study provides the full picture.
What Your Report Looks Like
The final report your doctor receives is organized into several sections. It opens with your symptoms and the reason the study was ordered. Next comes the sleep architecture summary: total time in bed, total time actually asleep, how long it took you to fall asleep, and the percentage breakdown of each sleep stage. The breathing section lists your AHI, your oxygen desaturation index, and your lowest oxygen level. Limb movement data, heart rhythm findings, and body position summaries follow.
The report ends with a clinical interpretation and recommendations. Depending on the findings, those might include a CPAP pressure setting, a referral for an oral appliance, a recommendation for positional therapy, or further testing. If your AHI was elevated, the report often specifies whether a split-night study was performed, meaning CPAP was introduced partway through the same night to find an effective pressure.