Definition reprinted with permission from Culebras, Antonio, Clinical Handbook of Sleep Disorders, Butterworth-Heinemann
A series of measurements of the interval from "lights out" to sleep onset that is used in the assessment of excessive sleepiness. Subjects are allowed a fixed number of opportunities to fall asleep during their customary awake period. Excessive sleepiness is characterized by short latencies. Long latencies are helpful in distinguishing physical tiredness or fatigue from true sleepiness.