The science of postprandial somnolence.
You did it again. You ate all the things. If you're the average American, that means you've consumed about two days worth of calories in one meal.
You stumble through the kitchen, groaning like a tranquilized elephant. Before you make it to a corner where maybe you can just curl up and die, you bump into an unmanned sofa. You plop down, unbutton your pants to give your food baby some space, and drift off into your annual Thanksgiving food coma.
It's only 7 PM, and you can't keep your eyes open. What's happened to you?
You're experiencing what scientists refer to as postprandial somnolence—extreme lethargy and sleepiness after a heavy meal. We've been using the term "food coma" in its place for a while, but we officially adopted it this February when Oxford Dictionary added it to its list (along with "cyberespionage" and "death stare").
Drowsiness after a heavy meal is a typical part of feasting. It's nothing new, and it's not exclusively American. It may not even be unique to Thanksgiving. "Today, there's not much of a difference between food we eat regularly and feasting," says food historian Amy Bentley, an associate professor of food studies at New York University. We drink booze. We eat pies. And we pass out. But how?
The first thing you did on Thanksgiving was show up. There was food, so you ate. You ate a lot. Turkey, stuffing, sausage balls, sweet potatoes, mashed potatoes, casseroles, and pies: you shoved them all into your mouth, chewed them up, and let them all drop down to your gut for digestion. "We don't stop when we're comfortably full. We wait until we're so full, we don't even know what we're doing," said nutritionist Lisa Young.
We used to think the reason for postprandial somnolence was because our blood left our brains
You may not have known what you were doing, but your body did. We used to think the reason for postprandial somnolence was because our blood left our brains and moved down to our mesenteric vessels—the tubes that supply our intestines and colon. With little blood to oxygenate the brain, it lost energy, and we felt drowsy. But that's not likely the case: our brain and our heart are pretty good at staying oxygenated. In a 2003 study, German researchers tracked blood flow in the body after twenty men ate a heavy meal. There was no difference in how much blood reached the brain.
In the past fifteen years our culture started to obsess about chemicals in the brain, and another idea became popular: turkey, which contains a chemical called tryptophan, induced food comas. When we ate turkey, tryptophan entered the blood and moved to our brain where it converted to serotonin, a chemical that does a lot of things, including making us sleepy.
But turkey has no more tryptophan than any other meat, said Young. The sleepiness we experience after a big meal is more likely due to carb-heavy, fatty foods with high glycemic indexes, such as pumpkin, stuffing, and potatoes.
Foods with a high glycemic index increase the amount of glucose, the sugar we need for energy, in our blood. When we have too much glucose, our bodies send out insulin to level things out and store that energy for later. Insulin triggers the release of serotonin and melatonin—two chemicals our bodies produce when we're sleepy. A 2007 study in The American Journal of Clinical Nutrition showed that consuming foods with high glycemic indexes made participants fall asleep faster.
A newer theory expands on this idea. In a in 2009 paper, two Korean scientists, Sang Woo Kim and Byung In Lee, outline how the gut and brain talk to one another to tell us it may be best to nap after overeating. After a feast, our gut sends out signals that it's satiated to our brain. The arucate nucleus, a part of the brain associated with appetite regulation, takes these signals and sends them to the ventromedial hypothalamus, or "satiety center." When the signal hits this spot in the brain, another part of the brain shuts down: an area that coordinates arousal called the lateral hypothalamic area.
The message: you're full; now rest—save your energy for later.
Bentley says Thanksgiving became a license to excess after Norman Rockwell's Freedom From Want solidified modern-day assumptions of Thanksgiving food. So you took your license, and you used it on casseroles, pies, and booze.
"Is it so wrong to eat a feast every once in a while?" Bentley asks. It's just a food coma. You're not going to die. So take it easy and exercise your freedom to excess.