Hormonal secretion are tightly controlled by the time of day due to an inbuilt biological clock in human body. This rhythmic secretion is controlled by:

- Suprachiasmatic nucleus of hypothalamus is said to be the link between external environment and body's biological clock/ circadian rhythm, - We see day & night, this image falls on retina |One set of fibers are sent to suprachiasmatic nucleus | superior cervical ganglionFibers are sent to pineal gland. |Pineal gland accordingly secretes melatonin- Melatonin secretion levels determine hormonal secretions & their circadian rhythm.Eg: steroid levels - highest at early morning & low in evening.