I still don't know what the heck it's for. —William Dement, Stanford Medicine News Centre Many theories have been proposed for the function of sleep. But there is no consensus among sleep experts about what sleep's purpose is. From an evolutionary perspective, sleep is strange behavior that equates to a period of inactivity and reduced vigilance in which an animal may be vulnerable to environmental threats. Biologists want to identify what it is about sleep that enhances the survival of the individual — the adaptive function of sleep. There must be a fundamental reason for not continuing to pursue other biological drives such as eating and reproduction. Sleep researchers believe that there must be an essential need that sleep satisfies, and they are not confident that it has been identified. If sleep does not serve an absolute vital function, then it is the biggest mistake the evolutionary process ever made. —Allan Rechtschaffen via Tom Chivers, The Guardian or via Scullin and Bliwise, Perspectives on Psychological Science The main reasons that researchers believe this essential function must exist are the following: There is no compelling evidence of animals that do not display some form of sleep-like behavior. Most animals demonstrate a compensatory response to sleep deprivation. Lack of sleep can lead to negative consequences like the intrusion of sleep into wakefulness, cognitive impairment, metabolic changes, and even death. Sleep is a basic biological need that is as essential as food or water, so sleep is a biological drive similar to hunger or thirst. However, the search for a single essential or core function for sleep may never be resolved, because sleep is likely to have several functions that can vary across species.