Polysomnography is a sleep study. It measures your sleep cycles and stages by recording brain waves (EEG), electrical activity of muscles, eye movement, breathing rate, blood pressure, blood oxygen saturation, and heart rhythm.

A specially trained health care provider will directly observe you while you sleep.

There are two states of sleep: non-rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep.

REM sleep is associated with dreaming. Your body muscles (except your eyes and lungs) do not move during this stage of sleep.

NREM sleep has four stages distinguishable by EEG waves.

REM sleep alternates with NREM sleep approximately every 90 minutes. A person with normal sleep usually has four to five cycles of REM and NREM sleep during a night.

Polysomnography is usually done in a sleep study center. You will lie on a bed in the test center. The test may be carried out during the night so that 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 electrodes are recorded while you are awake with your eyes closed and during sleep. The time taken to fall asleep is measured as well as the time to enter REM sleep.

Monitors to record your heart rate and breathing will be attached to your chest.

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

Do not take any sleeping medication and do not drink alcohol or caffeine-containing beverages before the test.