Circadian rhythms are the 12 hour cycles of alertness and tiredness that we all experience daily. Chronobiologists, or scientists that study circadian rhythms, have found that light and darkness trigger a rhythm of physical, mental, and behavioral changes that occur every 24 hours in most organisms, even plants and microbes!
Circadian rhythms are controlled by a cluster of approximately 20,000 nerve cells called the suprachiasomatic nucleus, or SCN. The SCN is located in the hypothalmus in the brain above the optic nerve area. Light is the main switch that turns specific genes on and off in order to regulate the organism's circadian cycles. These cycles include sleep/wake, body temperature, hormones, and others.
Jet lag sufferers can attest to the power of circadian rhythms when traveling through multiple time zones. It usually takes days for an individual's body clock to reset.