There is a blood-brain barrier between the human brain and the blood. Any substance that plays a role in sleeping must be able to pass through this barrier to be effective. Milk contains an amino acid called tryptophan that can cross the blood-brain barrier. It produces the hormone serotonin that induces sleep
As the skin temperature rises, falling asleep speeds up, so drinking a cup of hot milk is like taking a hot bath, which can speed up falling asleep.
people think that drinking milk before going to bed is a very effective way to promote sleep.