pineal gland

The pineal gland is a tiny structure deep in the center of your brain, about the size and shape of a grain of rice, named after the pine cone it vaguely resembles. Its main job is to act like a chemical clock for your body: when the world around you grows dark, it releases a hormone called melatonin into your bloodstream, and that rising tide of melatonin is the signal that tells the rest of you it is time to feel sleepy.

It does this by listening, indirectly, to your eyes. Light landing on your retina travels along a chain of nerve connections that eventually reaches the pineal gland, telling it whether it is day or night. In bright light the gland stays quiet; in darkness it switches melatonin production on. Because the gland tracks not just the daily light–dark cycle but also how long the nights are across the year, it helps many animals time seasonal behaviors such as breeding, hibernation, and the growing of a winter coat. In humans its rhythm is gentler, but it remains a key conductor of the daily sleep–wake cycle, which is why bright screens late at night, or jet lag from crossing time zones, can throw your sleep off by confusing this little gland.