Sleep: Sleep is a vital physiological need. Sleep is necessary to maintain alertness and performance, positive mood, and overall health and well-being. Each individual has a basic sleep requirement that provides for optimal levels of performance and physiological alertness during wakefulness. On average, this is 8 hours of sleep in a 24-hour period, with a range of sleep needs greater than and less than this amount. Losing as little as 2 hours of sleep will result in acute sleep loss, which will induce fatigue and degrade subsequent waking performance and alertness.

Principles and Guidelines for Duty and Rest Scheduling in Commercial Aviation, NASA Technical Memorandum 110404 (May 1996)

Individual Differences In Sleep Requirements. Many of the studies described above showed that there appears to be a considerable variability in individual sleep needs. Thus, the eight-hour sleep requirement represents the average of sleep needs, but does not take into account of the needs of those individuals who require additional sleep and who represent a fair percent of the population. .