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If eating makes you tired, you’ve got something in common with most people—and, for that matter, with most living things. Researchers have turned up evidence of “postprandial sleepiness,” also known as a food coma, in insects, snakes, worms and rats.

“The conservation of this behavior across species suggests that it’s really important for something,” says William Ja, an associate professor of neuroscience at the Scripps Research Institute in Florida who has studied this food coma phenomenon.

Some experts have hypothesized that animals—humans included—have built-in “vigilance signals” that keep them awake and alert when hungry. These signals help them locate and acquire food. It follows that once an animal (or a human) has eaten a lot, these vigilance signals dissipate and are replaced by feelings of fatigue.

Others have theorized that post-meal changes in blood circulation could explain why eating makes some people sleepy. Blood flow to the small intestine “dramatically increases” after a person eats, says Dr. Tomonori Kishino, a professor of health science at Japan’s Kyorin University. And as blood is pumped into the gut to fuel digestion, a corresponding drop in blood flow to the brain could trigger feelings of sleepiness, he says.

Some past research into this hypothesis concluded that blood flow to the brain does not change after a person eats a meal. But some of Kishino’s recent work found that, among people who skipped breakfast, one measure of cerebral blood flow plummeted after they ate lunch. “Skipping breakfast could therefore place a heavy burden on the body after lunch by causing greater changes in [blood flow],” he says. This could lead to sleepiness.

While scientists are still figuring out exactly why food comas happen, they’ve started to home in on some factors that may contribute to post-meal fatigue.

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Eating a big meal may be one trigger. Ja’s research on fruit flies suggests that meal size is a “strong driver” of post-meal sleepiness. So too are meals loaded with salt or protein. But why? He says one long-held idea is that sleep somehow aids digestion. One of his not-yet-published studies—again, on fruit flies—found that sleep changes the way the insects absorbed certain macronutrients, including protein. “This would support the idea that post-meal sleepiness affects gut nutrient absorption,” he adds.

Ja is quick to point out that his work may not translate to humans. But some of his findings—like the idea that certain foods are more likely than others to cause fatigue—dovetail with some recent research on people.

A small 2018 study of truck drivers found those who ate diets rich in vegetables and fats from foods like olive oil and dairy tended to experience less post-meal sleepiness than those eating “Western” diets heavy in processed meat, fast food and soft drinks. “Our results suggested that a healthy diet produces low sleepiness during the day,” says Claudia Moreno, one of the authors of that study and a faculty researcher at the University of São Paulo School of Public Health in Brazil. Her study points to some older research that suggests heavy fat or carbohydrate intake could potentially trigger sleepiness by disrupting the body’s natural circadian sleep rhythms.

Meanwhile, a 2018 study found that a high-fat, high-carb meal led to both sleepiness and an uptick in some inflammatory markers, especially among obese adults. But there’s still a lot of uncertainty and contradiction when it comes specific foods and their effects on post-meal fatigue. “Some human studies show an effect after eating, but others don’t,” Ja says.

His research in flies, he says, helps explain why a lot of the food-coma research on humans is so inconclusive. “We could see [the observed effects] because we used hundreds of flies and thousands of meals,” he says, “but these numbers are obviously much harder and more expensive to replicate in humans.

If you want to prevent a food coma, the best advice is to “eat smaller meals,” he says. This tactic may be especially effective at lunchtime. Predictable shifts in the body’s circadian rhythms tend to make people feel drowsy in the afternoon, so if you’re the type who eats a big lunch, you may be in for a double whammy.

Moreno’s research indicates that eating healthy, vegetable-centric meals could also help curb your post-meal fatigue. But the fact is, experts are still teasing out all the ins and outs of food comas.

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