Outline what melatonin is, where it is secreted, and what its effects are.
a) Melatonin is a hormone secreted by the pineal gland that regulates sleep-wake cycles.
b) Melatonin is a neurotransmitter secreted by the brainstem that controls muscle movement.
c) Melatonin is a digestive enzyme secreted by the small intestine that aids in digestion.
d) Melatonin is a hormone secreted by the adrenal glands that regulates stress response.