Introduction to Mammalian Circadian Clock System

Organisms on Earth are dictated by circadian changes (e.g., sunlight, humidity, and temperature) in the environment caused by the planet’s rotation around its own axis. All forms of life have evolved biological clocks to adapt (and to synchronize) their physiology and behaviors to circadian environmental changes. This adaptation results in circadian rhythms (with a period of ~24 h) in many physiological and behavioral processes such as the sleep–wake cycle, body temperature, energy metabolism, cognitive performance, and hormonal release. Perturbation of circadian rhythms is associated with various pathologic conditions, including cancers, metabolic syndromes, cardiovascular diseases, sleep disorder, and depression. On the other hand, many diseases present circadian rhythms in flares of symptoms. Thus, it is of great interest to investigate circadian rhythms in depth, which would help to find means to combat diseases and to optimize drug treatment. In this chapter, we introduce mammalian circadian clock system and discuss the role of circadian clock in human health.