Summary: Data from recent laboratory studies indicate that nocturnal sleep periods reduced by as little as 1.3 to 1.5 hours for 1 night result in reduction of daytime alertness by as much as 32% as measured by the Multiple Sleep Latency Test (MSLT). Other data document that 1) 17%-57% of normal young adults have MSLT latencies of £ 5.5 minutes, whereas £ 50% have MSLT values of ³ 10 minutes and 2) 28%-29% of young adults reported normally sleeping £ 6.5 hours on each weeknight. More extensive reduction of daily sleep amount is seen in nightshift workers. A minimum of 2%-4% of middle-aged adults have hypersomnolence associated with sleep apnea. Together, these data show that significant sleep loss exists in one-third or more of normal adults, that the effects are large and replicable and that similar effects can be produced in just 1 night in the laboratory. In light of the magnitude of this sleep debt, it is not surprising that fatigue is a factor in 57% of accidents leading to the death of a truck driver and in 10% of fatal car accidents and results in costs of up to 56 billion dollars per year. A recent sleep extension study suggests that the average underlying sleep tendency in young adults is about 8.5 hours per night. By comparison, the average reported sleep length of 7.2-7.4 hours is deficient, and common sleep lengths of £ 6.5 hours can be disastrous. We must recognize the alertness function of sleep and the increasing consequences of sleepiness with the same vigor that we have come to recognize the societal impact of alcohol.