… a simple definition of sufficient sleep is a sleep duration that is followed by a spontaneous awakening and leaves one feeling refreshed and alert for the day. — National Sleep Foundation
The National Sleep Foundation (NSF) in the United States regularly updates its sleep duration recommendations. The recommendations are derived from a systematic review of the scientific literature followed by a systematic voting procedure involving a panel of sleep experts. The NSF recognizes that there is a significant amount of variability between individuals. They, therefore, provide ranges rather than a specific number of hours. They also acknowledge that duration is only one measure of sleep quality.
There are two types of scientific study the recommendations are based on:
- Studies of acute sleep restriction: in which individuals who are deprived of a certain amount of sleep undergo a series of tests to determine the effects. There are several tests that can be performed such as blood tests, sleep latency tests, cognitive or other performance-based tests.
- Population studies: usually large surveys in which the health of individuals are correlated to the amount of sleep that they report getting.