Answer:
Melatonin.
Explanation:
The melatonin hormone is been secreted by the pineal. And this happens in hours of darkness in a mammalian body especially during the sleep-wake cycle. Also the compound named pinoline is also said to be produced in the pineal gland, and it is one of the beta-carbolines. Though it is not totally proven and still arguable.
But still on the melatonin's case, it functions also in the central nervous system. Its working process is been explained as when light sensitive nerve cells found in the retina denotes light and transfers this signal to the suprachiasmatic nucleus and other parts of the nervous system.
