pineal gland

The pineal gland is a pea-sized organ buried deep in the center of the brain that acts as the body's nightfall sensor. Although it sits in darkness, it learns whether it is day or night through nerve signals that originate in the eyes, and it announces nighttime to the rest of the body by secreting the hormone melatonin.

Information about light reaches the pineal by a roundabout route: light detected by the retina is relayed to the brain's master clock, the suprachiasmatic nucleus, and from there through the sympathetic nervous system to the pineal. In the dark the pineal ramps up melatonin synthesis from the neurotransmitter serotonin; in the light, production is shut off. The gland is thus a transducer that converts a neural signal about light into a hormonal signal about time.

Named for its pinecone shape, the pineal gland fascinated philosophers, who once speculated it housed the soul. In modern physiology its role is well defined but narrower: it is the chief source of circulating melatonin and a key link between the light cycle and the body's internal rhythms.