Glossary · Sinh học
Circadian rhythm
Some of the physical, mental and behavioural changes an organism goes through over a 24-hour cycle. Light and dark have the biggest influence, but food intake, stress, physical activity, social environment and temperature also affect it.
The system that controls circadian rhythms is the biological clock, made of proteins encoded by thousands of genes that switch on and off in a specific order; a master clock coordinates all the biological clocks in an organism. In vertebrates, including humans, the master clock is in the brain. In humans it is the suprachiasmatic nucleus (SCN), a large group of nerve cells that receives information about light directly from the eyes. The SCN controls production of the hormone melatonin according to how much light the eyes receive — in the evening the master clock tells the brain to make more melatonin, and we feel sleepy — and synchronizes the rhythms of organs and tissues throughout the body.
Circadian rhythms direct many functions, from daily changes in wakefulness to body temperature, metabolism and hormone release. Together with sleep homeostasis, they regulate when we are awake and when we sleep.