Bio-clock:

Organisms have evolved to co-ordinate their activities with the day-night cycle caused by the Earth's rotation. Direct responses to light or darkness are important but, in addition, biological clocks have evolved to time biological processes. "Circadian" rhythms (from 'circa'-about, 'dies'-a day) are the result of the best-characterised of these biological clocks, which times events that occur once per day. Even in the absence of environmental time cues, circadian rhythms persist with a period close to 24 hours. The circadian clock regulates many aspects of metabolism, physiology and behaviour, in humans and many other organisms.

The 4-gram (about the weight of a ruby-throated humming bird) hypothalamus houses the Supra chiasmatic nucleus. This brain part sits behind the eyes, directly below the thalamus. See figure, colored yellow. The hypothalamus controls every hormone gland in the body and thereby changes vital body functions: blood pressure, temperature, metabolism, and how much adrenaline pumps through the system.

The hypothalamus causes body operation to ebb and flow in a daily rhythm like the tides.