What is Polysomnography?


Polysomnography (PSG) is a test used to diagnose sleep disorders, usually done at the hospital. It measures how well you sleep by recording your brain waves, the oxygen level in your blood, heart rate and breathing, as well as eye and leg movements during the study. This test can help your doctor to find out whether you have a sleep disorder and how severe it is.

The test is important because untreated sleep disorders can contribute to a number of medical conditions including heart disease, high blood pressure and stroke. Sleep disorders have also been linked to an increased risk of injury, such as having a fall and road traffic accidents, can cause a poor quality of life, and need to be properly diagnosed and treated.

In addition to helping diagnose sleep disorders, PSG may be used to help adjust your treatment plan if you have already been diagnosed with a sleep disorder.


PSG monitors your sleep stages and cycles to identify if or when your sleep patterns are disrupted and why.

The normal process of falling asleep begins with a sleep stage called Non-Rapid Eye Movement (NREM) sleep. During this stage, your brain waves, as recorded by electroencephalography (EEG), slow down considerably. Your eyes don’t move back and forth rapidly during NREM, in contrast to later stages of sleep. After an hour or two of NREM sleep, your brain activity picks up again, and Rapid Eye Movement (REM) sleep begins. Most dreaming occurs during REM sleep.

You normally go through multiple sleep cycles a night, cycling between NREM and REM sleep in about 90 minutes. Sleep disorders can disturb this sleep process.

Your doctor may recommend PSG if he or she suspects you have:

  • Sleep apnoea or another sleep-related breathing disorder, where your breathing repeatedly stops and starts during sleep.
  • Periodic limb movement disorder, where you involuntarily flex and extend your legs while sleeping. This condition is sometimes associated with restless legs syndrome.
  • Narcolepsy, where you experience overwhelming daytime drowsiness and sudden attacks of sleep in this condition.
  • REM sleep behaviour disorder, which involves acting out dreams as you sleep.
  • Unusual behaviours during sleep. Your doctor may perform this test if you do unusual activities during sleep, such as walking, moving around a lot or rhythmic movements.
  • Unexplained chronic insomnia. If you consistently have trouble falling asleep or staying asleep, your doctor may recommend PSG.


PSG is a non-invasive, painless test. The most common side effect is skin irritation caused by the adhesive used to attach test sensors to your skin.


You will be advised to avoid drinks or food containing alcohol or caffeine during the afternoon and evening before having the test. Alcohol and caffeine can change your sleep patterns, and they may make symptoms of some sleep disorders worse.

Your doctor will ask you about your regular medication. Make sure you tell them about all the medicines you’re taking, including over-the-counter products. Some medicines can affect the sleep study results.

You should try to sleep well for two nights before having the test. Napping the afternoon before the sleep study is discouraged.


You arrive at the hospital in the evening for PSG and stay overnight. You may bring items you use for your bedtime routine, and you can sleep in your own nightclothes.

The room where PSG is done is a single room, with its own bathroom. It is dark and quiet during the test.

After you get ready for bed, one of the nurses will place sensors on your scalp, temples, chest and legs using a mild adhesive, such as glue or tape. A small clip will also be placed on your finger or ear to monitor the level of oxygen in your blood.

While you sleep, the following parameters will be recorded:

  • Brain waves
  • Eye movements
  • Heart rate
  • Breathing pattern
  • Blood oxygen level
  • Body position
  • Chest and abdominal movement
  • Limb movement
  • Snoring and other noise you may make as you sleep.

During the night, if you need assistance, you can call the nurse by pressing the bell.

Although you probably won’t fall asleep as easily or sleep as well at the hospital as you do at home, this usually doesn’t affect the test results. A full night’s sleep isn’t required to obtain accurate PSG results.

In the morning, the sensors are removed, and you may leave the hospital. You’re given an appointment for explanation of the result with the doctor who recommended the test. You can return to your usual activities after PSG.

Some PSG studies are conducted during the day, they include:

  • Multiple Sleep Latency Test (MSLT): this study is done to see how sleepy you are during the day. The MSLT is used most often to diagnose narcolepsy (inability to stay awake during the day) and see what might be causing your excessive daytime sleepiness;
  • Maintenance of Wakefulness Test (MWT): this study is done to see how well you can remain awake when you are in a situation that makes it easy to get sleepy.


The measurements recorded during PSG provide a great deal of information about your sleep patterns. For example:

  • Brain waves and eye movements during sleep can help your health care team assess your sleep stages and identify disruptions in the stages that may occur due to sleep disorders such as narcolepsy and REM sleep behaviour disorder;
  • Heart and breathing rate changes and changes in blood oxygen that are abnormal during sleep may suggest sleep apnoea;
  • Frequent leg movements that disrupt your sleep may indicate periodic limb movement disorder;
  • Unusual movements or behaviours during sleep may be signs of REM sleep behaviour disorder or another sleep disorder.

You will not receive a diagnosis right away. The information gathered during the PSG is evaluated first by the nurse, who uses the data to chart your sleep stages and cycles to generate a medical report. That information is reviewed by your doctor.

At a follow-up appointment, your doctor reviews the results with you. Based on the data gathered, your doctor will discuss any treatment or further evaluation that you may need.

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