I had always heard that warm milk will help you sleep. The claim is that milk, like turkey, contains tryptophan, the sleep-inducing amino acid.
But researchers find that eating protein-rich foods, including milk, can decrease the ability of tryptophan to enter the brain, which would be necessary to have any effect on sleep. So it's unlikely that the tryptophan in milk has an sleepy side effects. Does this mean the ability of warm milk to lull you to sleep is just an old wives' tale?
Observing how GoatSpawn likes to have a little nip of milk before nodding off for a nap, even when she isn't hungry, I suspect that the idea of warm milk before bed works, not on some biological basis, but on a warm, fuzzy memory of being nursed to sleep as a baby.