A physical exam and exhaustive medical history are essential for proper diagnosis of narcolepsy. However, none of the major symptoms is exclusive to narcolepsy. Several specialized tests, which can be performed in a sleep disorders clinic or sleep lab, usually are required before a diagnosis can be established. Two tests that are considered essential in confirming a diagnosis of narcolepsy are the polysomnogram (PSG) and the multiple sleep latency test (MSLT). The PSG is an overnight test that takes continuous multiple measurements while a patient is asleep to document abnormalities in the sleep cycle. A PSG can help reveal whether REM sleep occurs at abnormal times in the sleep cycle and can eliminate the possibility that an individual's symptoms result from another condition. The MSLT is performed during the day to measure a person's tendency to fall asleep and to determine whether isolated elements of REM sleep intrude at inappropriate times during the waking hours. As part of the test, an individual is asked to take four or five short naps usually scheduled two hours apart.