When heading to bed, people often do a variety of rituals to help them prepare for a restful night's sleep, such as taking a warm bath or doing nighttime yoga. But what about the time-honored tradition of drinking a cup of warm milk before getting under the covers? Is there any scientific evidence that drinking a tall glass will make you sleepy?
Tryptophan is an essential amino acid; that means the body can't produce it, so people have to get it from their diet, according to the National Library of Medicine. the tryptophan in milk has sleep-promoting properties. Once you ingest tryptophan by drinking milk or eating foods such as eggs, turkey, fish, soy or peanuts your body uses it, among other things, to create the brain chemical serotonin which in turn gets converted into the sleep-aiding hormone melatonin.
Source: Live Science