Polysomnography, also called a sleep study, is a test used to diagnose sleep disorders. Polysomnography records your brain waves, the oxygen level in your blood, heart rate and breathing, as well as eye and leg movements during the study.Polysomnography usually is done at a sleep disorders unit within a hospital or at a sleep center. You'll be asked to come to the sleep center in the evening for polysomnography so that the test can record your nighttime sleep patterns. Polysomnography is occasionally done during the day to accommodate shift workers who habitually sleep during the day.In addition to helping diagnose sleep disorders, polysomnography may be used to help adjust your treatment plan if you've already been diagnosed with a sleep disorder.