The Multiple Sleep Latency Test (MSLT) is a diagnostic tool for the objective measurement of sleep propensity during the day. It is used for the differential diagnosis of narcolepsy, hypersomnia and other disorders with increased daytime sleepiness. Using electroencephalography, electromyography of the chin, and electro-oculography, this sleep test measures the average latency to fall asleep and the occurrence of REM sleep under ideal sleep conditions in a darkened room and the request to fall asleep. The test is performed during the day following polysomnography the previous night and consists of five standardized recordings, each 2 h apart. Medication, sleep deprivation, and stimulating substances or activities may affect the results of the test. In 2021, the MSLT guideline was updated by the American Academy of Sleep Medicine (AASM). This article presents the updated AASM guideline and comments on it.