How to Read a Sleep Study Report

Quick answer: Start with the study type and total sleep time, then find the AHI, the oxygen desaturation data and the sleep architecture. The AHI is the headline breathing number, but it is not the whole report: oxygen drops, arousals, sleep efficiency and body position all change what the result means. A report is interpreted alongside your symptoms, not instead of them.

First, identify what kind of study it was

A report from an in-lab polysomnogram and a report from a home sleep apnea test are not interchangeable. The lab study records brain waves, eye movements, muscle activity, breathing, oxygen, heart rhythm, body position and often video. A home test usually records breathing, effort, oxygen and heart rate, with far fewer channels.

Look at the first page for the study type, the date, the total recording time and the total sleep time. A lab study measures sleep directly. A home test often estimates sleep time from the breathing signal, which can make the AHI less precise, especially if you were awake for part of the night. Our comparison of a home sleep test and an in-lab study explains the trade-offs.

Sleep timing and architecture

These numbers describe how you slept, not just how you breathed:

• Total sleep time (TST): the minutes actually spent asleep, not the hours in bed.
• Sleep efficiency: total sleep time divided by time in bed, as a percentage. Around 85% or higher is often considered normal in adults, though age and circumstances matter.
• Sleep onset latency: how long it took to fall asleep. Around 10 to 20 minutes is typical; very long or very short values can be meaningful.
• Wake after sleep onset (WASO): minutes awake after first falling asleep.
• REM latency: time from sleep onset to the first REM period. It is usually around 90 minutes, but it shifts with sleep deprivation, alcohol, antidepressants and other factors.

The report may also show the percentage of time spent in N1, N2, N3 and REM sleep. A typical adult night is roughly 5% N1, 45–55% N2, 15–25% N3 and 20–25% REM, but there is wide normal variation. A single odd percentage is rarely a diagnosis on its own.

The breathing numbers: AHI, RDI and ODI

This is where most people start, and where most confusion lives.

Term	What it counts	Why it matters
Apnea	A complete or near-complete stop in airflow for at least 10 seconds	Obstructive events show effort; central events show no effort
Hypopnea	A partial reduction in airflow for at least 10 seconds, usually with an oxygen drop or an arousal	The exact definition varies, which is one reason AHI can differ between labs
AHI	Apneas plus hypopneas per hour of sleep	The main severity number for sleep apnea in adults
RDI	AHI plus respiratory effort-related arousals (RERAs)	Can be higher than AHI when subtle narrowing fragments sleep
ODI	Oxygen desaturations per hour, usually 3% or 4% drops	Shows how often oxygen fell, even if breathing events were not scored
T90	Percentage of sleep time with oxygen saturation below 90%	A measure of how long oxygen was low, not just how low it went

For adults, an AHI below 5 is generally considered normal, 5 to 15 mild, 15 to 30 moderate, and above 30 severe. Those bands are useful for triage, not for describing how you feel. Two people with the same AHI can have very different symptoms and risks.

Oxygen numbers deserve their own look

The report usually gives a baseline oxygen saturation, a lowest value (nadir), and sometimes the percentage of time below 90% or 88%. A nadir of 85% during a short event is different from spending 30% of the night below 90%. Both may matter, but they are not the same problem.

Oxygen numbers can look worse in people with lung disease, during high altitude, or with certain heart conditions. They can also look better than the breathing problem suggests if the events are mostly arousals without much desaturation. Ask the clinician to interpret the oxygen data with your overall health in mind.

Arousals, limb movements and snoring

An arousal is a brief shift to lighter sleep or wakefulness, usually lasting a few seconds. The arousal index counts them per hour. A high number can explain unrefreshing sleep even when the AHI is modest. Arousals are not always caused by breathing; noise, pain, reflux and limb movements can do it too.

The periodic limb movement (PLM) index counts repetitive leg movements during sleep. A value above 15 per hour in an adult is often flagged, though the clinical significance depends on whether the movements fragment sleep or cause symptoms. In children, the threshold is lower. Iron status and medications are common things a clinician will review.

Snoring is usually recorded as a percentage of sleep time or a simple yes/no. It is a symptom, not a diagnosis. Plenty of people snore without apnea, and plenty of people with apnea do not snore loudly.

Position and REM can change the story

Many reports break the AHI down by body position and by sleep stage. Obstructive sleep apnea is often worse on the back. If your overall AHI is 12 but your supine AHI is 40, the average hides a clinically important pattern. Positional therapy may be worth discussing, though it is not right for everyone.

REM-related apnea can also be missed by an overall average, because REM sleep is a smaller share of the night. Some people have relatively normal breathing in NREM sleep and much worse events in REM. The report may show a separate REM AHI. Ask whether that pattern is present.

Heart rhythm and other findings

A lab study includes an ECG. The report may mention sinus arrhythmia, premature beats, pauses, atrial fibrillation or other rhythm changes. Some are normal variants; others need follow-up with a cardiologist. Do not ignore a flagged arrhythmia just because the sleep clinic is focused on breathing.

The report may also note bruxism, sleep talking, periodic breathing, central events, Cheyne-Stokes respiration, or movements during sleep. These findings are clues, not always diagnoses. A single night in an unfamiliar bed can also produce a poor study that does not represent your usual sleep.

If the study included a CPAP titration

A titration study is used to find a pressure that keeps the airway open. The report will show the pressure or pressure range, the residual AHI at that setting, leak if recorded, and the sleep stages achieved. A good titration is not just the number that eliminated events; it also considers whether you slept in REM and on your back, and whether the pressure was tolerable.

If the report recommends a fixed pressure, ask why. If it recommends an auto-adjusting range, ask how the range was chosen. Pressure settings are a clinical decision, not something to change on your own based on one night.

What the report cannot tell you

A sleep study is a snapshot. It cannot measure how tired you feel, how well you function at work, or whether you will tolerate treatment. It also cannot rule out insomnia, restless legs, depression or a circadian rhythm problem just because the breathing numbers are normal.

A home test is especially limited: it usually cannot score arousals or RERAs reliably, and it may underestimate the AHI if sleep time is overestimated. A negative home test in someone with strong symptoms may need a lab study or a repeat test. Our guide to sleep apnea treatment options explains what happens after a diagnosis.

Children are scored differently

Pediatric sleep medicine uses different normal values. An AHI of 1 or 2 may be significant in a child, and the severity bands are not the adult ones. Children may also have more subtle symptoms, such as behavioural problems, poor growth or bedwetting, rather than loud snoring. A pediatric sleep physician should interpret the study. Do not apply adult cut-offs to a child's report.

Questions to ask about your report

1. What was my total sleep time and sleep efficiency?
2. What was my AHI, and what definition of hypopnea did the lab use?
3. What was my RDI, and were RERAs scored?
4. How low did my oxygen go, and for how long was it below 90%?
5. Was my apnea worse on my back or in REM sleep?
6. Were there arousals or limb movements that could explain my fatigue?
7. What would you recommend, and what would change your mind?

Ask for a copy of the full report, not just the summary letter. You may need it for a second opinion, insurance, or a new clinic. If you are looking for a clinician to review it with you, start with our sleep doctor directory.

Frequently Asked Questions

What is a normal AHI on a sleep study?

In adults, an AHI below 5 events per hour is generally considered normal. Mild sleep apnea is usually 5 to 15, moderate is 15 to 30, and severe is above 30. Children have different thresholds, and symptoms matter as much as the number.

What does ODI mean on a sleep report?

ODI is the oxygen desaturation index: the number of times per hour your oxygen saturation drops by a set amount, usually 3% or 4%. It is a marker of how often breathing was disrupted, but it does not by itself diagnose sleep apnea.

Can a sleep study report be normal if I still feel exhausted?

Yes. A normal breathing study does not rule out insomnia, circadian rhythm disorders, restless legs, depression, thyroid disease, iron deficiency or other causes of fatigue. Bring the report and your symptoms to a clinician who can look at the whole picture.

Why is my AHI different from the number on my home test?

Home tests use fewer sensors and often estimate sleep time, so they can under- or overestimate the AHI. The hypopnea definition can also differ. A lab study with EEG is more precise, especially when the home result does not fit your symptoms.

What is a dangerous oxygen level during sleep?

There is no single cut-off that applies to everyone. A brief drop to the mid-80s is common in sleep apnea; spending a long time below 90% is more concerning. Your clinician will interpret the nadir and the time below 90% together with your heart and lung health.

How do I get a copy of my sleep study?

Ask the sleep lab or the ordering clinic for the full report, including the raw data if you need a second opinion. In most countries you are entitled to your records, though a small fee may apply. Keep a digital copy.

This article is general information, not medical advice. It does not replace an assessment by a qualified clinician who can review your history, medications and test results.