There are a few reasons some people think warm milk helps you fall asleep. First off, milk contains tryptophan, the same ingredient that makes everyone sleepy after Thanksgiving dinner. In the body, tryptophan is converted to the sleep-inducing hormones serotonin and melatonin. Besides that, warm milk is, well, warm. The few degrees that drinking it might raise your body temperature to is often enough to start the body’s slowdown response we have on warm days making us relax and sleep. In addition to those two things there is also anecdotal evidence that the warm milk reminds us of when we were infants, hence we “sleep like a baby.”
Most other beverages we enjoy warm usually have caffeine in them, which inhibits sleep (hot chocolate, coffee, black tea), but many herbal teas are just as good as warm milk, and in fact are recommended for the lactose-intolerant.
So basically a glass of warm milk at bedtime may be comforting, helping you relax a bit and fall asleep faster, but there’s little scientific evidence that it actually helps you fall asleep. In fact, there are no specific foods that scientifically can help you sleep better.