Circadian Cycles: The Biological Clock
A biological cycle, or rhythm, that is approximately 24 hours long is called a circadian cycle. This biological clock is actually a cluster of neurons in the hypothalamus. Body temperature, the hormone melatonin, and alertness each fluctuate on a circadian cycle. Any disruptions in this wake-sleep cycle causes fatigue and irritability.
A phenomenon closely related to disruptions of this wake-sleep cycle is jet lag when the circadian cycle is thrown off its usual track.
An overview of the human biological clock.
