Melatonin is made from serotonin, so serotonin levels drop as it gets used up in melatonin manufacture. The quickest way to raise serotonin levels again is to send more tryptophan into the brain, because serotonin is made from tryptophan. The quickest way to send more tryptophan into the brain is to secrete insulin from the pancreas, because insulin causes the body to absorb other chemicals similar to tryptophan, thus removing competitors for the channels that take tryptophan into the brain. And the quickest way to secrete insulin is to eat a carbohydrate snack.
