Human body has rhythmic fluctuations in their function on a circadian cycle. Circadian rhythm is controlled by:

Within the hypothalamus, a specific cell group, the suprachiasmatic nucleus, functions as an intrinsic clock. Cells within this nucleus show circadian rhythms in metabolic and electrical activity, and in neurotransmitter synthesis, and appear to keep the rest of the brain on a day-night cycle. Ref: Waxman S.G. (2010). Chapter 9. Diencephalon. In S.G. Waxman (Ed), Clinical Neuroanatomy, 26e.