Melatonin is secreted by the pineal gland in the absence of light, signaling darkness.

“we need that Darkness at night because when you give the brain the signal of Darkness it releases effectively a brake pedal that brake pedal has normally been applied by way of light on the release of that spigot of meel melatonin and when you take the brake pedal off it starts starts pumping out into the brain you can also then of course probably reverse engineer this trick in the morning and this is another component of why you've been I think so such a wonderful advocate for light in the morning it does many things but one of the things that it does is reapply that break on melatonin and therefore you lose the signal to your brain of Darkness that's what melatonin in some ways is doing we often call it the hormone of Darkness or the vampire hormone”