Pineal gland

The pineal gland is an endocrine gland, meaning one of the glands that make hormones and release them into the blood. It sits in the back of the cranial fossa, inside the brain, and it is also called the epiphysis cerebri. Under the microscope it is made of cells called pinealocytes, plus the supporting cells that go with them.

What it makes is melatonin, and it makes it from serotonin, which is its precursor, that is, the molecule it starts from. The key to this gland is not how much it produces but when: production rises with darkness and falls when there is light.

That is the source of the pattern that almost everyone has noticed without knowing its name. Melatonin in the blood is low during daylight hours and climbs until it peaks at night. That rise and fall is what helps modulate the circadian sleep rhythm, the roughly one-day cycle the body uses to alternate between being awake and being asleep. It is a clock that does not tell time with hands, but with the amount of a substance in the blood: little by day, a lot by night.