Melatonin is a hormone that rises in the evening and helps signal that night has arrived.

The longer version

As light falls, the master clock signals the pineal gland to release melatonin into the blood, and levels climb through the evening before peaking in the middle of the night. The hormone acts as a chemical signal of darkness. Levels fall again toward morning, and the rhythm tracks the clock rather than the amount of sleep obtained.

It does not force sleep so much as mark the appropriate time for it. Secretion is suppressed by bright light, especially blue-rich light, which is part of why evening screens have drawn research attention. Supplements are widely sold, but the evidence for shifting the clock is strongest for timed light exposure and consistent scheduling. Many animals show the same nightly rhythm.

Where this entry sits

This collection gathers curious details about sleeping and dreaming, from nightly brain stages and body clocks to naps, yawns and unusual animal habits. It also covers how bedtime customs have changed across centuries, without offering any advice. The full list sits on the Sleep and Dreams page, where every entry is listed in order.