Polysomnography (Sleep Study)

What is polysomnography? This is a medical study monitoring you as you sleep. It is also known as a “Sleep Study” or “PSG.” This testing provides a comprehensive recording of the physiology during sleep.

What does PSG monitor? A sleep study monitors:

  1. Brain waves (EEG)
  2. Breathing rate and depth
  3. Electrical activity of the muscles (EMG)
  4. Eye movements (EOG)
  5. Heart rate and rhythm (EKG)
  6. Body positions
  7. Blood oxygen (pulse oximetry)
  8. Snoring

Why is PSG performed? To diagnose possible sleep disorders, including:

  1. Sleep Apnea Syndrome
  2. Narcolepsy
  3. Periodic Limb Movements Disorder/Restless legs
  4. Insomnia
  5. Parasomnias
  6. REM Behavior Disorder

How is PSG performed? Typically, the study is done at night, in a special sleep center. You will arrive a few hours before bedtime and sleep in a bed at the center, in an individual, quiet and comfortable room. You will have monitors placed on your chin, scalp, chest, legs, nostril, finger,and outer edges of your eyelids. A specially trained health care provider will monitor you as you sleep.

How to prepare for a sleep study? Arrive at the requested time and do not take any sleeping medications, alcohol, caffeinated beverages or stimulants before the test.

What is a “split night” sleep study? Occasionally, when your doctor suspects sleep apnea and wants to test your response to a pressurized breathing apparatus (called nasal CPAP), you might have a split night test, half the night to diagnose apnea and the other half to test the treatment. The advantage is less cost to you (only one night instead of two) and convenience of only one night of testing. The disadvantages are less time to make the diagnosis and less time to test the nasal CPAP treatment.

How is PSG interpreted? After you complete the study, your results will be “scored.” Scoring consists of looking at a number of different parameters across 30 second epochs of your sleep. The doctor will assess:

  1. Sleep Onset Latency- how long it took you to get to sleep after they turn out the lights (as determined by the brain waves). Normally, it is under 20 minutes.
  2. Sleep efficiency- the number of minutes asleep divided by the total number of minutes in bed, times 100. Normally, 85-90% or more.
  3. Sleep stages, 1 through 4 and REM (rapid eye movement) sleep- the total times and percentages of each.
  4. Arousals- sudden shifts in brain wave activity.
  5. Breathing irregularities, especially apneas or hypopneas- Apneas are complete or near complete stopping of airflow (breathing) for at least 10 seconds, followed by arousal and/or 3% or more drop in oxygen saturation. Hypopneas are 50% drops in airflow for at least 10 seconds, followed by arousal and/or 3% or more drop in oxygen saturation.
  6. Cardiac (heart) rhythm abnormalities
  7. Leg movements
  8. Body position
  9. Oxygen saturations- drops of 3% or more, drops below 90% (danger zone) and total time (or percentage of time) below 90%.


  1. National Heart Lung and Blood Institute
  2. National Institutes of Health