Brain
Pineal Gland
Timekeeper of the brain
Overview
The pineal gland is a small, pea-shaped endocrine gland situated in the epithalamus, deep in the brain center. It secretes melatonin—primarily in darkness—to regulate circadian rhythms and the sleep-wake cycle. Descartes famously called it "the seat of the soul." It is one of the few midline brain structures not duplicated bilaterally.
Function
- Produces and secretes melatonin in response to darkness
- Regulates circadian rhythms and seasonal timing
- Modulates reproductive hormone cycles
- Influences immune function via melatonin signalling
- Coordinates internal timing with external environment
Key Facts
- Melatonin production peaks around 2 AM in complete darkness
- The pineal calcifies with age—detectable on X-rays
- Descartes believed it was the interface between mind and body
- It is evolutionarily derived from a light-sensing organ (third eye) in some reptiles