Sleep timing is controlled by the circadian clock, sleep-wake homeostasis, and in humans, willed behavior. The circadian clock is an inner timekeeping, temperature-fluctuating, enzyme-controlling mechanism that works in tandem with adenosine, a neurotransmitter that inhibits many of the bodily processes associated with wakefulness.
Adenosine is created over the course of the day, with high levels of adenosine leading to sleepiness. In diurnal animals, sleepiness occurs as the circadian element causes the release of the hormone melatonin and a gradual decrease in core body temperature.
The timing is affected by one’s chronotype, yet it is the circadian rhythm that determines the ideal timing of a correctly structured and restorative sleep episode. The need for sleep as a function of the amount of time elapsed since the last adequate sleep episode must be balanced against the circadian element for satisfactory sleep.
Along with corresponding messages from the circadian clock, this tells the body it needs to sleep. Sleep offset, or awakening, is primarily determined by circadian rhythm. A person who regularly awakens at an early hour will generally not be able to sleep much later than his or her normal waking time, even if moderately sleep-deprived.