What Is Polysomnography (PSG)?

Medically Reviewed by Neha Pathak, MD on June 30, 2021

Polysomnography (PSG) is a big word that simply means “sleep study.” Sleep specialists use it to see how you sleep overnight. A polysomnogram records your brain waves while you sleep. It also records your oxygen levels, breathing, heart rate, and how your eyes, arms, and legs move while you sleep or try to sleep at night.

Your doctor may use this test if they think you have a sleep disorder such as narcolepsy or sleep apnea. They might suggest you take it along with another test called a multiple sleep latency test (MSLT). The MSLT is sometimes called a daytime nap study. It looks at how fast you fall asleep during the day when you should be awake.

Preparing for Polysomnography

You’ll have to stay at a hospital, sleep clinic, or hotel room overnight for your study. To get you ready, your doctor will likely ask you to take the following steps:

  • For the most part, follow your normal schedule for the day.
  • Don’t take naps.
  • Avoid caffeine, alcohol, and sedatives.
  • Don’t use hair sprays or other substances that might affect the recordings.

Take things along with you like you would for any overnight. This includes:

  • Pajamas
  • Toothbrush and other toiletries
  • A book to read
  • Clothes for the next day

Be ready to answer questions about your sleep habits. Otherwise, come ready to settle in, stay for the night, and sleep as best you can.

What to Expect During Polysomnography

After you get to the hospital, hotel, or sleep center, your sleep technologist will apply sensors to measure your brain waves, heart rate, and movements during sleep. The sensors won’t hurt and you’ll still be able to move around.

You’ll have some time to read or watch TV before it’s time for bed. When it’s time to go to sleep, the lights will go off. Your technologist will stay nearby to help as needed and keep an eye on you overnight.

Chances are you won’t sleep as well as you would at home. That’s normal. Most people don’t sleep as well in a new place as they do at home. As long as you get some sleep, your sleep doctors will get some good information to find out what’s causing your sleep trouble.

In the morning, they’ll remove the sensors. Then they’ll likely ask you to fill out a form about how you slept.

Polysomnography Results

When people fall asleep normally, they enter non-rapid eye movement sleep (NREM sleep) first. Just like it sounds, your eyes don’t move quickly in NREM sleep. Your brain waves slow down.

After a couple of hours, brain activity picks up and your eyes start to move more. Doctors call this rapid eye movement (REM) sleep. You dream the most in REM sleep.

Normally, your sleep should go back and forth between NREM and REM sleep throughout the night in cycles that last about 90 minutes. If you have a sleep disorder, your sleep patterns might look different. The polysomnography results can help doctors find out if you have a sleep disorder. They’re used to help diagnose problems including:

Your sleep study results will show how your sleep cycles changed while you slept and anything abnormal. They’ll also find any other issues you have with breathing or movement during sleep. For example:

  • If you have narcolepsy or another REM sleep behavior disorder, your sleep cycle patterns will be disrupted. People with narcolepsy often enter REM sleep more quickly than other people do.
  • Changes in heart rate, breathing, and oxygen levels may mean you have sleep apnea.
  • Lots of leg movements may suggest you have periodic limb movement disorder.
  • Other unusual movements or results may be signs of another sleep disorder.

Based on these results, your doctor can give you a diagnosis and help you with a treatment plan. Usually one sleep study is enough. Sometimes your doctor might suggest you do another, since results can vary from one night to the next. If they need more information, they also may suggest you have a multiple sleep latency test or other tests, too.

Show Sources


Mayo Clinic: “Polysomnography (sleep study).”

Stanford Health Care: “Polysomnogram,” “Multiple Sleep Latency Test (MSLT).”

American Academy of Sleep Medicine: “Sleep Study.”

Medscape: “Polysomnography.”

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