Multiple sleep latency test（ MSLT） is an objective evaluation and a standard tool of the daytime sleepiness. Polysomnography（ PSG） was used to conduct multiple sleep latency test. The sleep frequency， average sleep latency， and the latency and frequency of rapid eye movement（ REM） sleep were analyzed. The severity of sleepiness could be displayed by MSLT with the length of the sleep latency， which is more objective and repeatable. Now MSLT has been widely used in clinical diagnosis of narcolepsy， the severity evaluation of daytime sleepiness and obstructive sleep apnea-hypopnea syndrome， and the efficacy evaluation of drugs such as psychostimulants.