Sleep timing is controlled by the circadian clock, sleep-wake homeostasis, and in humans, within certain bounds, willed behavior. Biologically, the most important circadian clock currently known to science is a dense cluster of neurons in the Suprachiasmatic nucleus , a part of the brain directly above the optic chiasm, where the optic nerves cross on their paths from the two eyes to the visual cortex. This clock measures the time of day, primarily based on input from outside light signals.
An organism whose circadian clock exhibits a regular rhythm corresponding to outside signals is said to be entrained; the rhythm so established persists even if the outside signals suddenly disappear. The actual circadian clock has a cycle length of 25 hrs so the clock resets every day depending on the environment. The clock exerts a constant influence on the body, effecting continuous sinusoidal…
