Main organ for biological clock is

Hypothalamic control of circadian rhythm: A large number of physiologic rhythms ( body temperature, sleep-wakefulness, coisol secretion among a host of others ) in the body run to a cyclic pattern that is approximately 25 hours long. These rhythms are entrained to 24 hours due to environmental signals such as light and darkness. These signals are called zeitgebers. The suprachiasmatic nucleus of the hypothalamus is believed to be responsible for the rhythmicity of these circadian cycles(Ref: Guyton & Hall Textbook of medical physiology- a south Indian edition 2014 pg no: 812)