Circadian biorhythms occur in many different organisms from prokaryotes to higher mammals, and nearly all physiological and behavioral processes in animals possess daily variations that are controlled by circadian timing. The sleep/wake cycle in animals is the most obvious and easily observable circadian biorhythm and is an indispensable function of a healthy life. Chronic phase shifts in the biological clock cause circadian disruption that can lead to increased mortality, increased risk for cancer, sleep disorders, and various other conditions or diseases.