The biological clock of our body is the

The pineal gland secretes melatonin, the hormone that regulates circadian rhythms (sleep-wake cycles), earning it the name 'biological clock'. Melatonin production increases in darkness and decreases in light, synchronising our internal body clock with the external day-night cycle. The adrenal gland secretes cortisol and catecholamines (stress response), pituitary is the 'master gland' controlling other endocrine glands, and thyroid regulates metabolism - none are the biological clock.