All animals and plants have a built-in circadian rhythm, which is adjusted or entrained to the environment by external cues, known as Zeitgebers (a German word meaning “time-givers"), the most important of which is daylight. The brain’s internal circadian clock (also known as the biological clock, body clock, circadian pacemaker, circadian system, circadian oscillator, etc), which is centred in the hypothalamus region of the basal forebrain, uses these Zeitgebers to naturally synchronize or reset itself each day to within just a few minutes of the Earth’s 24-hour rotation cycle (the word “circadian” comes from the Latin words meaning “about a day”).
The brain’s circadian clock regulates sleeping and feeding patterns, alertness, core body temperature, brain wave activity, hormone production, regulation of glucose and insulin levels, urine production, cell regeneration, and many other biological activities. The most important hormones affected by the circadian clock, at least insofar as they affect sleep, are melatonin (which is produced in the pineal gland in the brain, and which chemically causes drowsiness and lowers body temperature) and cortisol (produced in the adrenal gland, and used to form glucose or blood sugar and to enable anti-stress and anti-inflammatory functions in the body).
