What Is a Sleep Study (Polysomnography)?

Here is what a polysomnography sleep study measures overnight, and how it helps evaluate sleep-related problems.

The purpose of a sleep study

A sleep study, or polysomnography, records brain waves, breathing, blood oxygen levels, and muscle movement together overnight, to objectively evaluate sleep-related conditions such as sleep apnea, restless legs syndrome, and REM sleep behavior disorder.

How the test is performed

A standard sleep study is usually done overnight in a sleep lab, with several sensors attached to the body while the person sleeps as naturally as possible. In some cases, a simplified home sleep testing device can be used instead, though whether that's appropriate depends on what's being evaluated.

Measuring brain waves (EEG)

Electrodes attached to the scalp record brain wave activity, which shows how sleep moves between stages, light sleep, deep sleep, and REM sleep, and how often brief awakenings occur during the night.

Measuring muscle and eye movement (EMG and EOG)

Sensors record jaw and leg muscle activity (EMG) along with eye movement (EOG), which together help identify REM sleep, leg-movement-related issues, and teeth grinding.

Measuring breathing

Sensors placed near the nose and mouth, and on the chest and abdomen, record airflow and breathing effort, which is used to detect apnea, breathing pauses, and hypopnea, shallow breathing.

AHI: the Apnea-Hypopnea Index

AHI counts the number of apnea and hypopnea events per hour of sleep, and it is one of the main reference numbers used to classify the severity of sleep apnea. Interpreting it accurately still requires a clinician's overall judgment.

Measuring blood oxygen levels

A sensor, typically on a finger, tracks changes in blood oxygen saturation throughout the night, which shows whether breathing irregularities are actually affecting oxygen levels in the blood.

How results are used

Sleep study results are generally interpreted together with a patient's history and physical exam, rather than used to make a diagnosis on their own, and serve as a key reference point for deciding on further care.

A comprehensive test that records several signals at once

As the name suggests, polysomnography records brain waves, muscle activity, eye movement, breathing, and blood oxygen levels simultaneously over a night of sleep. That combination is what allows it to capture sleep quality and breathing patterns together in a way no single measurement could on its own.

General information, not a diagnosis

This page explains what a polysomnography test measures and how it's performed, as general educational information. It is not a diagnosis or treatment recommendation for any specific sleep disorder; if you suspect a sleep problem, talk with a healthcare provider about whether testing is appropriate.

Frequently Asked Questions

Does an unfamiliar sleep environment affect the results?

It can. Some people find it harder to fall asleep naturally in an unfamiliar lab setting than they would at home. Clinicians account for this when interpreting results, and a repeat test is sometimes recommended if needed.

Does an AHI number alone diagnose a sleep disorder?

No. AHI is an important reference number, but a clinician reaches a diagnosis by considering it together with symptoms, oxygen level changes, and other test findings; a single number on its own isn't meant to be self-diagnosed.