If you’ve ever had an unstoppable urge to crash on the couch after eating — the most common example is after a hearty Thanksgiving dinner — then you’ve experienced what scientists call “postprandial somnolence,” or in layperson’s terms, a food coma.
There are a number of popular theories about what causes people to feel tired and sluggish after eating, including the Thanksgiving turkey. Turkey (and a number of other foods, like chicken, fish, cheese, yogurt, and eggs) contains an amino acid called tryptophan, which is the alleged culprit behind your tiredness. But experts say suggesting the tryptophan in turkey contributes to food-coma feelings is a bit of a leap, and reads like such: In your body, tryptophan is converted to niacin, a B-vitamin that helps the body create serotonin, which is a mood-boosting hormone, that can help regulate sleep.
According to David Levitsky, PhD, professor of nutritional sciences at Cornell University, it’s true that turkey contains tryptophan, and that tryptophan is a precursor to serotonin. But there’s no scientific basis to the assumption that eating turkey would cause someone to feel tired, primarily because there’s not enough tryptophan in turkey to impact the brain.
“There’s a whole area out there called fantasy nutrition, where people take a little knowledge and blow it out of proportion,” he says. “The amount of tryptophan we’re talking about in turkey is so trivial compared to what is necessary to alter brain serotonin, that it doesn’t really make any difference,” he says.
Levitsky says the sheer amount of food we eat on occasions like Thanksgiving is more likely to trigger a food coma — and it has to do with the autonomic nervous system, the same system involved in the fight-or-flight and rest-and-digest responses.
The stomach and intestines contain stretch receptors, neurons that detect digestion and send messages to the brain about our physiological state. The act of eating puts the autonomic nervous system into parasympathetic mode (also known as the rest-and-digest response) so the body can conserve energy for digestion.
When the body is in this state, it dilates blood vessels in the stomach, causing blood to move from muscles and the brain and into the gut. Levitsky says it’s the blood leaving the brain that might induce feelings of sleepiness.
Technically, this automatic, energy-zapping response occurs anytime you’re eating, no matter the amount or the occasion. But it’s probably more noticeable when you’re eating a lot in a short period of time, like on Thanksgiving. “The more we eat, the greater the parasympathetic dominance will be, and the sleepier we’re going to get,” Levitsky says.
While research confirms the role of the autonomic response in eating, William Ja , associate professor of neuroscience, recently studied flies to uncover more about the specific mechanisms at play during food comas. Outside of his office at the Scripps Research Institute in Jupiter, Florida, he has a cartoon of fruit flies gathered around a table eating a Thanksgiving dinner.
Why flies, and not animals or humans?
Practicality is one reason. Flies aren’t just cheap and abundant; they also only live two months, so it’s easy to study them over their entire lifetimes. But the main reason, Ja says, is that fruit flies are easy to genetically manipulate so they mimic human brains — scientists have been using them for more than a hundred years to study human diseases and even discover new drugs. In the case of food coma research, Ja could easily activate and turn off hunger circuits in the fruit flies’ brains.
In his 2015 study, Ja and his colleagues analyzed hundreds of fruit flies over thousands of meals, feeding them small and large portions containing different macronutrients and then observing their sleep habits. They not only discovered that flies that were fed more food slept more, but that the nutrient content of the meals impacted how much the flies slept.
Smaller meals containing more protein or salt caused a similar effect as the larger meals that weren’t high in protein and salt, suggesting that what we eat — like protein-rich turkey and the high-salt sides that come with it — might impact postprandial sleep as much as how much we eat.
It’s probably not the tryptophan or mashed potatoes, because if you had that exact same meal for breakfast or dinner, you’d probably feel pretty good.
To figure out why flies got sleepier depending on what they ate, Ja and his colleagues allowed a group of flies a single meal (they ate proteins, like amino acids and yeast, salt, and sugar, like cornmeal or molasses), then took half and sleep deprived them. The sleep-deprived flies absorbed more of the nutrients they just ate compared to the flies allowed to sleep right after a meal.
Ja suspects changes in absorption play an important role in the flies’ health: Both protein and salt at high levels are actually toxic to flies and other insects. “So maybe postprandial sleep [in humans] is a good thing because it helps excrete protein and salt when you’ve had too much of them in a meal,” he says.
Ja’s findings point researchers in a direction for understanding the physiological reasons people get sleepy after meals, but the phenomenon is still relatively unstudied in humans. That’s mainly because it’s hard to analyze a person’s sleepiness after a hearty dinner when a number of other factors determine how sleepy someone is: The time of day, how much someone slept the night before and the quality of sleep, and a person’s emotional state could all contribute to their response to a meal.
Rafael Pelayo, MD, clinical professor of psychiatry and behavioral Sciences at the Stanford Center for Sleep Sciences and Medicine, offers up a much simpler hypothesis about food comas: that the timing of when you eat might be more important than the amount or type of food you consume. A big breakfast generally doesn’t make you sleepy, and if anything, dinner often gives people a second wind.
“It’s probably not the tryptophan or mashed potatoes, because if you had that exact same meal for breakfast or dinner, you’d probably feel pretty good,” he says. “On Thanksgiving, you typically eat earlier, during the afternoon when you already have a lull in your alertness level.”
Pelayo says this timing has to do with our natural circadian rhythm. Throughout the day, we tend to get sleepier — like during the “afternoon slump.” The inverse is also true: We’re usually more alert during breakfast and dinner times because thousands of years ago, predators were more likely to attack during dawn and dusk.
There are a lot of speculations and some preliminary research about what causes people to be tired after eating, but with the lack of human studies it’s hard to know exactly what causes people to be tired after eating. Even Ja admits his preliminary findings are just that — preliminary — and probably just one part of a much bigger, complex puzzle.
For example, as Pelayo mentions, the same behavioral variables that make it difficult to study human eating and sleeping could also contribute to the uncontrollable urge to nap after Thanksgiving dinner. On top of eating a lot more than usual at an unusual time, many people drink alcohol with their meals, which could make them sleepier. The stress of holiday travel and waking up early to get things ready might also lend themselves to sleep deprivation.
So while it could be that the Thanksgiving meal, or eating in general, has its own unique sleep-inducing qualities, it’s likely that our behavior on the occasion brews up a perfect storm for sleepiness — and a cozy afternoon nap.