Why does warm milk make you sleepy but cold milk doesn't?
Dear Straight Dope:
Why does warm milk help you sleep but cold milk doesn't?
At least two things go into this:
1) Tryptophan: the same ingredient that makes everyone sleepy after Thanksgiving dinner is present in milk.
2) Warmth: the few degrees that this drink might raise your body temperature is often enough to trigger the slowdown response we have on warm days.
In addition there is anecdotal evidence that the warm milk reminds us of the time when we were infants, hence "sleep like a baby."
Most other beverages we enjoy warm tends to have caffeine, which inhibits sleep (coffee, hot chocolate/cocoa, leaf tea), but many herbal teas are as good as warm milk and in fact are recommended for the lactose-intolerant.