An internal mechanism which operates in an approximately 24 hour cycle is called a circadian rhythm.
All animals produce endogenous circadian rhythms. Circadian rhythms are responsible for the maintenance of the internal body mechanisms synchronized with the external environment.
Some of the mechanisms which show circadian rhythms are,
Hormones- GH, Cortisol, Prolactin
Kidney- Blood filtration
There are number of oscillators in the organs of our body which control the circadian rhythms of the each organ. But the main oscillator in the suprachiasmatic nucleus of the hypothalamus controls all these slave oscillators and synchronizes to work with the internal environment.
Suprachiasmatic nucleus (anterolateral and dosomedial parts) generates circadian rhythms by light sensitive gene expression. Information about the light travels from the retina to the Suprachiasmatic nucleus by the retinohypothalamic pathway.Suprachiasmatic nucleus also controls the pineal gland. Pineal gland secretes melatonin, which is a hormone responsible for increasing sleepiness. Melatonin taken in the afternoon works as a sleep aid and can phase advance the circadian rhythm. Melatonin is a very powerful antioxidant known. Also it has an up regulatory effect on cellular immunity. Treatment methods like acupuncture may be associated with re-setting the slave oscillators throughout our body.