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
The total amount of tryptophan in a small portion of milk is not enough to cause the body's hormone levels to fluctuate greatly. Only by drinking a lot of milk will the effect of sleeping aid be good.
people think that drinking milk before going to bed is a very effective way to promote sleep.