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A Comprehensive Study for Diagnosing Sleep Disorders

Alternative Names:

Sleep study; Polysomnogram; Rapid eye movement studies; Split night polysomnography; PSG

Polysomnography is a sleep study. A sleep study monitors you as you sleep, or try to sleep.

There are two states of sleep:

  • Rapid eye movement (REM) sleep is when you dream. Your muscles (except your eyes and breathing muscles) do not move during this stage of sleep.
  • Non-rapid eye movement (NREM) sleep has four stages that can be detected by brain electrical activity (EEG) waves. REM sleep alternates with NREM sleep about every 90 minutes. A person with normal sleep usually has four to five cycles of REM and NREM sleep during a night.
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A sleep study measures your sleep cycles and stages by recording:

  • Air flow in and out of the lungs during breathing
  • Blood oxygen levels
  • Body position
  • Brain waves (EEG)
  • Breathing effort and rate
  • Electrical activity of muscles
  • Eye movement
  • Heart rate

How the Test is Performed


The most common type of sleep study is performed at a special sleep center. You will be asked to arrive about 2 hours before bedtime. You will sleep in a bed at the center.

Many sleep centers have rooms that are similar to hotel rooms, so that you are in a comfortable bedroom. The test is often done during the night so that your normal sleep patterns can be studied.

The health care provider will place electrodes on your chin, scalp, and the outer edge of your eyelids. These must remain in place while you sleep.

Signals from the electrodes are recorded while you are awake (with your eyes closed) and during sleep. The time it takes you to fall asleep is measured, as well as the time it takes you to enter REM sleep.

Monitors to record your heart rate and breathing will be attached to your chest. These also must stay in place while you sleep. A specially trained health care provider will observe you while you sleep and note any changes in your breathing or heart rate. The number of times that you either stop breathing or almost stop breathing will be measured.

In some sleep study centers, a video camera records your movements during sleep.


Portable sleep study devices that can be used in the home instead of at a sleep center are available to help diagnose sleep apnea. You go to a sleep center to pick up the device, or a trained therapist comes to your home to set up the device.

Portable devices are best used when:

  • You are under the care of a sleep specialist
  • Your sleep doctor thinks you have obstructive sleep apnea
  • You do not have other sleep disorders
  • You do not have other serious medical conditions, such as heart disease or lung disease

How to Prepare for the Test

Do not take any sleep medicine and do not drink alcohol or caffeinated beverages before the test.

Why the Test is Performed

The test is done to diagnose possible sleep disorders, including obstructive sleep apnea (OSA). Often, OSA is suspected because of the following symptoms:

  • Daytime sleepiness (falling asleep during the day)
  • Loud snoring
  • Periods of breath holding, followed by gasps or snorts
  • Restless sleep

Other sleep disorders that polysomnography may be used to diagnose are:

  • Narcolepsy
  • Periodic limb movements disorder (moving the legs often during sleep)
  • REM behavior disorder (a condition in which people physically "act out" their dreams)
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Normal Results

A sleep study keeps track of:

  • How often you stop breathing for at least 10 seconds (called apnea)
  • How often your breathing is partly blocked for 10 seconds (called hypopnea)

Results are most often reported using the Apnea-Hypopnea Index (AHI). An AHI of less than 5 is considered normal.

A normal test result also shows normal patterns of brain waves and muscle movements during sleep.

What Abnormal Results Mean

Apnea-Hypopnea Index (AHI) results are used to diagnose obstructive sleep apnea.

  • 5 - 15 is mild sleep apnea
  • 15 - 30 is moderate sleep apnea
  • More than 30 is severe sleep apnea

The sleep specialist must also look at other findings from the sleep study, as well as the medical history and physical exam to make the diagnosis and decide on treatment. A sleep study can also help diagnose narcolepsy.


Adapted from Medline plus, a service of the US National Library of Medicine and NIH

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