The Scientists: Marcello Iriti, Ph.D., professor of plant biology at Milan State University, who discovered melatonin in grapes; and Vandana Sheth, a registered dietitian nutritionist and spokesperson for the Academy of Nutrition and Dietetics

The Answer: If you’re feeling sleepy (not just tipsy) after a glass of wine, it’s most likely due to the combination of alcohol and the fact that most people are hitting the bottle after a long day when we’re already winding down, says Sheth. Many people swear that they feel especially tired after a glass of red, compared with white, and there’s some evidence of a scientific explanation, though no study has actually been done to show it.

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The flesh, skin and seeds of grapes, it turns out, contain melatonin in a form that’s very similar to the hormone we produce in the body to go to sleep at night. Something in the winemaking process, probably related to the yeast, seems to increase melatonin levels, giving wine a higher concentration than grapes or non-alcoholic grape juice. And since red wine is made with skin, while white wine is not, it stands to reason that there would be more in a merlot than a chardonnay. That said, it’s not a huge amount either way, and it’s hard to know what effect, if any, it’s really having.

We don’t know how much the brain needs to make you drowsy, but depending on the yeast used for fermenting and the varietal—some may be more melatonin-y than others—a glass of red contains around 60 to 120 nanograms of melatonin. A 1 mg melatonin pill has about 10,000 times more than that.

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