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Melatonin is the starting official
The official gathers the runners at the starting line and begins the race, then plays no part in the race itself. Sleep is a whole different set of brain mechanisms and chemicals. That is why melatonin is not an effective sleep aid once a person is stable in a new time zone.
Darkness matters because it removes the brakes from melatonin, which has to rise to time the normal onset of healthy sleep. Industrialized nations are a dark-deprived society, and electrical light at night, even though it is not as powerful as daylight, can keep the brake pedal on melatonin.
How does darkness trigger melatonin release?
Light entering the eye reaches the suprachiasmatic nucleus, the brain's master clock, which sends signals along a chain of nerves to the pineal gland. In darkness the gland releases melatonin into the blood. Light cuts the signal and melatonin falls. The gland's role was worked out in the 1950s when Aaron Lerner at Yale isolated melatonin from the pineal glands of cattle.
How much does room light cut melatonin?
In a 2011 study in the Journal of Clinical Endocrinology and Metabolism, Joshua Gooley and colleagues at Brigham and Women's Hospital exposed volunteers to ordinary room light in the hours before bed. The light suppressed melatonin and shortened its nightly duration by about 90 minutes compared with dim light. Nearly all participants showed some suppression. Their melatonin onset was delayed too.
Is melatonin high at night in nocturnal animals too?
Yes. Melatonin rises in the dark in both day-active animals such as humans and night-active animals such as mice and rats. The hormone signals night rather than sleep. What differs between species is how the rest of the brain responds to that signal.