The body uses two mechanisms to regulate sleep. One is the body clock, which attunes humans and animals to the 24 hour cycle of day and night. The other mechanism is the sleep ‹homeostat›: a device in the brain that keeps track of your waking hours and puts you to sleep when you need to reset. This mechanism represents an internal nodding off point that is separate from external factors. When it is turned off or out of use, sleep deficits build up. via University of Oxford.