Can drinking warm milk really help you fall asleep?

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? 

The answer, it turns out, is multifaceted. Milk contains a variety of amino acids, the building blocks of proteins, that may promote sleep in different ways. Moreover, if partaking in a warm glass of milk is soothing to you for personal reasons, that can help set the stage for a successful night’s sleep, experts told Live Science.

“[One] reason warm milk makes people sleepy is because it reminds you of the person who was kind enough to give it to you when you were younger,” said Michael Breus, a clinical psychologist, board certified sleep specialist in California and author of “Good Night: The Sleep Doctor’s 4-Week Program to Better Sleep and Better Health” (Dutton Adult, 2006). The calming association may help to lower pre-sleep anxiety, he said.

On the molecular level, the tryptophan in milk has sleep-promoting properties. 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. 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. 

Melatonin is usually produced by the brain in response to darkness and is involved in the regulation of the body’s natural circadian rhythm, or its 24-hour internal clock, according to a 2017 study in the journal Current Neuropharmacology. Serotonin, on the other hand, is known as the “happy hormone” and is known to induce both sleep and wakefulness, according to a 2002 paper published in the journal Sleep Medicine Reviews.

In theory, eating foods or drinking milk rich in tryptophan can make us feel drowsy, because the body converts it into hormones that enhance sleep. For that same reason, there is a popular myth that eating tryptophan-rich foods, like turkey at Thanksgiving dinner, is the reason people feel drowsy  after a big family dinner. But in reality, it would take a lot of tryptophan — way more than is in a glass of milk or a serving of turkey — to make a person feel lethargic. 

If you were to drink about 2 gallons (7.6 liters), it might make you feel sleepy, but “you would be pretty sick” from drinking such a high volume of milk, Breus told Live Science. You might even feel nauseous — 2 gallons of whole fat milk is equivalent to more than double the recommended calorie intake for an adult. 

Even if a person were to down that much milk, it’s unclear whether the high doses of tryptophan would be enough to make them feel sufficiently sleepy. Milk, after all, contains many other compounds that  compete to enter our brain through our blood. Lin Zheng and Mouming Zhao, food scientists at South China University of Technology, agree that the sleep-inducing effect of tryptophan in milk is limited.

Live Science