Why does warm milk serve as an effective sleep inducer for many people?

Milk contains tryptophan, an amino acid, which is the precursor of a brain transmitter, serotonin, which we know has sedative qualities. L-tryptophan supplements, which gained popularity as a sleeping aid, were banned by the Food and Drug Administration because they caused severe reactions in some users, including eosinophilia, an increase in the number of white blood cells. Earlier research confirmed that L-tryptophan did help many people get to sleep faster than a placebo. But does cow's milk contain enough tryptophan to induce sleepiness? No. Representatives of the dairy industry cited research that says it might take a dose as high as a half-gallon to provide any sleep benefits. And does warm milk work better than cold milk to induce sleep? It does if you believe it does, but none of the experts consulted could provide a single logical reason why it should.
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