All humans spend about one-third of their lives asleep. Despite the ubiquitous nature of sleep, the science of sleep is still in its infancy, though it is growing rapidly. Many studies have reported ways in which sleep is related to health and longevity; concurrently, research is showing that problems with sleep can be ameliorated with behavioral and medical interventions. It is clear that while it is possible to have good sleep but be in poor health, it is difficult to have poor sleep and be in good health. Additionally, sleep is a health behavior, in that (a) there is a substantial behavioral component that is under voluntary control; (b) this behavior is related to overall health in much the same way as other health behaviors, such as diet and exercise; and (c) this behavior exists in a social and cultural context. This entry introduces some of the basics of how sleep is regulated, how sleep can be disturbed, and how sleep disturbance affects health outcomes such as longevity, …