A neurophysiological technique that quantifies drowsiness as the speed of falling asleep at intervals across a day is used to identify patterns of sleepiness/alertness. The Multiple Sleep Latency Test (MSLT) reveals a daily biphasic organization of sleepiness that is affected in predictable ways by the length and continuity of nocturnal sleep on one or several nights, and by maturation, aging, sleep pathology, and drug ingestion. The systematic nature of these relationships provides impetus to efforts examining the neurobiological mechanisms subserving the delicate balance of sleep and wakefulness.