Melatonin

Melatonin is a hormone found in animals, plants, and microbes. In animals, levels of melatonin have a daily cycle. It drives the circadian rhythms of several biological functions.

Melatonin is produced in the pineal gland. The pineal gland is on the blood side of the blood-brain barrier.

It secretes melatonin "directly into the systemic circulation", thus melatonin is not affected by the blood-brain barrier

See also

To dig deeper: Wikipedia