Polysomnography is performed in a sleep laboratory while the patient sleeps, preferably at his or her usual sleeping time. The polysomnogram (PSG) objectively records sleep stages and respiratory events. It shows multiple channels of electroencephalogram (EEG), electrooculogram (EOG), electrocardiogram (ECG), nasal and oral airflow, abdominal, chest and leg movements and blood oxygen levels. A single part of a polysomnogram is sometimes measured at home with portable equipment, for example oximetry, which records blood oxygen levels throughout the night. Polysomnography is not routinely used in the evaluation of patients with insomnia or circadian rhythm disorders, except as needed to rule out other disorders. It will usually be a definitive test for sleep apnea.