melatonin

Melatonin is a hormone your brain makes at night that acts like an internal announcement: "it's dark out — time to wind down for sleep." It is produced by a tiny structure deep in the brain called the pineal gland, which is about the size of a grain of rice. As evening falls and light fades, the pineal gland starts releasing melatonin into the blood; levels climb through the night, peak in the small hours, and fade away by morning. Think of it less as a knockout drug and more as a gentle dusk signal that tells the rest of the body the night has begun.

What makes melatonin special is that it is a chemical clock-hand: its timing is set by light, not by how tired you are. A pacemaker in the brain (the master body clock) watches the light coming through your eyes, and only when that light drops does it allow the pineal gland to switch melatonin on. This is why bright screens or lamps at night can blunt the signal and make sleep harder to start, and why melatonin pills are often used to nudge a jet-lagged or shifted body clock back toward a normal schedule. Melatonin does not so much force sleep as it opens the gate for it — lowering alertness, cooling the body slightly, and telling your daily rhythms that the dark phase has arrived.