Daytime sleepiness is such a familiar thing for most Japanese, dozing off in a commuting train is regarded as completely normal. Sleepiness, however, can yield a lot of problems. Not only deteriorating efficiency of the work, it sometimes leads to traffic accidents, and also serves as a cause for disastrous accidents. Although excessive daytime sleepiness has been gradually known to Japanese health workers as one of the symptoms of sleep apnea syndrome, some patients cannot recognize sleepiness, and even neglect it, feeling others are sleepy as well. In this respect multiple sleep latency test is a valuable tool as an objective measures of daytime sleepiness, and commonly used procedures of this testing was described in details.