Sleep and circadian rhythms

The regulation of sleep is processed by the homeostatic physiology of the circadian rhythm, the sleep/wake cycle. Circadian rhythm is the 24-hour internal clock in our brain that regulates cycles of alertness and sleepiness by responding to light changes in our environment.
Circadian rhythm disorders, also known as sleep-wake cycle disorders, are problems that occur when your body's internal clock, which tells you when it's time to sleep or wake, is out of sync with your environment.