Melatonin

Melatonin is produced by the pineal gland in response to darkness, rising a couple of hours before habitual sleep and falling towards morning. It is a timing signal rather than a sedative: it tells the body when night is, rather than forcing sleep.

It matters because that distinction changes how it should be used. As a supplement its strongest evidence is in shifting circadian timing — jet lag, shift work, delayed sleep phase — where small amounts taken at the right time help. As a sleeping pill for ordinary insomnia its effect is modest at best.

It also matters that light suppresses it. Bright light, particularly blue-weighted light, in the couple of hours before bed measurably delays melatonin onset, which is the mechanism behind evening screen advice.