Sleep Study (Polysomnography)

What Is a Sleep Study?

A sleep study is a test that records data about you while you sleep. It’s usually done in a special lab. It’s also known as a polysomnogram (PSG) or polysomnography.

During the test, an EEG monitors your brain activity to identify sleep cycles and disturbances. A sleep specialist uses this as well as your movements, breathing, oxygen levels, and heart rate to find out whether you have a sleep disorder.

What Does a Sleep Study Test For?

You might have a sleep study to check for:

Types of Sleep Studies

There are four kinds of sleep studies.

Diagnostic overnight PSG monitors your general sleep and certain body functions, including breathing, oxygen levels in your blood, heart rhythms, and limb movements.

Diagnostic daytime multiple sleep latency test (MSLT) can help diagnose narcolepsy and check your degree of daytime sleepiness. It measures how quickly you fall asleep in quiet situations during the day. It also monitors how quickly and how often you enter the sleep stage called REM sleep. You’ll have this test the morning after a diagnostic overnight PSG.

Two-night evaluation PSG and CPAP titration. On the first night, you'll be monitored and evaluated. If the medical team finds that you have sleep apnea, you'll come back for a second night to figure out the right air pressure for CPAP (continuous positive airway pressure) treatment. It delivers air into your airways through a specially designed nasal mask.

Split-night PSG with CPAP titration. You’ll have this test if the medical team finds or suspects moderate or severe sleep apnea during the first part of the night's study. The second half of the test figures out the CPAP level needed to treat your apnea.


How to Prepare for a Sleep Study

What to do before

Your doctor will tell you whether to take your medications as usual or stop using them before the test. Don’t have any caffeine or alcohol the day of the test, because they can interfere with the results.

What to bring

Bring comfortable pajamas, a book or magazine, and a special pillow if you use one. You might be able to have a friend or family member there to make you more comfortable until bedtime.

What to Expect During a Sleep Study

During the study

You'll have a private bedroom in a sleep center or hospital. Technicians will sit nearby to monitor you while you sleep. Your room will have a video camera so they can see what’s going on and an audio system so you can talk with the technicians. Your room will have a private bathroom; just let the technicians know when you need to use it so they can take off the wires connecting you to the monitoring equipment.

All the equipment and monitoring may seem uncomfortable at first. But most people fall asleep without much trouble.

Equipment used

A technician puts sticky surface electrodes on your face, scalp, chest, and limbs. These send electrical signals, generated by your brain and muscle activity, to the measuring equipment. While you sleep, these signals digitally record your brain activity, heart rate, heart rhythm, and blood pressure.

Other sleep study tests and equipment

EEG (electroencephalogram) to measure and record brain wave activity

EMG (electromyogram) to record muscle activity such as face twitches, teeth grinding, and leg movements; it also helps pinpoint when you’re in REM stage sleep

EOG (electro-oculogram) to record eye movements; these movements help show when you’re in different sleep stages, especially REM sleep

EKG (electrocardiogram) to record heart rate and rhythm

Nasal airflow sensor to record airflow

Snore microphone to record snoring

After a sleep study

In the morning, the technicians remove the sensors attached to your skin, and you go back to your everyday activities.

It takes the sleep specialist some time to go over the hundreds of pages of data from the study. They’ll send the results to your doctor. Once your doctor reviews them, you’ll meet to talk about the findings and next steps.


Sleep Study Results

The data will include information about your sleep, such as:

  • How long you spend in each sleep stage
  • How often you wake up
  • Whether you stop breathing or have trouble breathing
  • Whether you snore
  • Body position
  • Limb movements
  • Unusual brain activity patterns

WebMD Medical Reference Reviewed by Nayana Ambardekar, MD on November 16, 2019



National Sleep Foundation: “How Does a Sleep Study Work?”

UpToDate: “Overview of polysomnography in adults.”

Johns Hopkins Medicine: “What Happens in a Sleep Study?”

Mayo Clinic: “Polysomnography (sleep study).”

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