A glass of warm milk is known to help you fall asleep. A reputation that seems to be age-old wisdom based on the fact that milk contains tryptophan, one of the essential amino acids that your body uses to make sleep-inducing brain chemicals such as seratonin and melatonin. But apparently, just consuming a glass of milk won’t make tryptophan to do its “sleep-inducing job.” Why? Because milk also includes other amino acids that decrease the ability of tryptophan to enter the brain.
“To have any soporific effect, tryptophan has to cross the blood-brain barrier. And in the presence of other amino acids, it ends up fighting ― largely unsuccessfully ― to move across,” explains Anahad O’Connor in a New York Times article.
Therefore it seems recommendable to combine a small amount of milk with an ample dose of foods high in carbohydrates, such as cookies. Carbohydrates trigger a rise in blood sugar levels, which in turn stimulate the release of insulin. Insulin facilitates the entry of tryptophan into the brain, and as a result it can make up for the counteracting effects of other amino acids found in milk.
So although taking a cookie and milk before bed has been a cliché used for many years, it seems that it isn’t just an old wives’ tale but it actually holds some scientific fact.
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Wurtman, R.J., Wurtman, J.J., Regan, M.M., McDermott, J.M., Tsay, R.H., & Breu, J.J. (2003). Effects of normal meals rich in carbohydrates or proteins on plasma tryptophan and tyrosine ratios The American Journal of Clinical Nutrition, 77 (1), 128-132