<prompt> 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:Supra optic nucleiPostero lateral nucleusVentro-lateral nucleusSupra-lateral nucleus  <response>: - Suprachiasmatic nucleus of hypothalamus is said to be the link between external environment and bodys 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. 