Answer:
Melatonin
Explanation:
Pineal gland is a tiny endocrine gland located in the brain. It secretes Melatonin which is released into the bloodstream. Melatonin is a hormone that regulates the body's daily (circadian) clock and is commonly used in human research to understand the body's biological time. There is a rhythm to the biology of the pineal gland. Melatonin is secreted according to the amount of light a person is exposed to. It varies based on changes in day length. Thus, referred to as both an endocrine clock.
