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
Carbohydrates such as rice have a better sleep aid effect.They stimulate the synthesis of insulin, allowing amino acids other than tryptophan to enter the muscle tissue, thereby making it easier for tryptophan to enter the brain.
people think that drinking milk before going to bed is a very effective way to promote sleep.