The urge to nap after a heavy meal like the Thanksgiving feast is commonly referred to as food coma. But what happens in the body to cause this post-feast dip — known as postprandial somnolence in the medical community — isn’t clear, according to nutrition and sleep experts.
One fact that isn’t under debate: Unlike popular belief, sleepiness isn’t caused by a lack of blood supply to the brain as blood rushes from the head to the stomach to aid with digestion.
The digestive tract, essentially one long muscle from end to end, does need additional blood flow when it begins to move and contract to process food, according to Lona Sandon, a clinical nutrition professor at the University of Texas-Southwestern in Dallas. But the blood tends to come from skin and skeletal muscle in our limbs, which tend not to be in use much anyway when we are sitting down and eating. The brain, on the other hand, is the most important organ in the body and therefore is protected and has adequate blood supply unless under duress, like with a head injury, she says.
The after-meal dragging feeling instead could be associated with changes in certain hormones induced by the food, says Ms. Sandon, who is also a spokeswoman for the American Dietetic Association. Or it could be as simple as the feeling of being weighted down by a large amount of food sitting in our stomachs, she says.
“When you just have this heaviness and this sensation of this bulk in your stomach, you just don’t want to move,” she says. “If you have a smaller meal where you feel satisfied but not stuffed, you don’t get sleepiness.”
Another possibility is that food coma isn’t related to food or digestion at all. Rather, it’s because our bodies are wired to feel tired during the afternoons and evenings when there is a natural dip in the circadian rhythm, the internal clock that regulates our sleeping and waking hours, says Robert Basner, director of Columbia University Medical Center’s sleep-studies program.
“I know people seem to think it’s due to a meal and it may have something to do with it, but usually people get sleepy around that time,” Dr. Basner says.
A person with a normal sleep-wake cycle will typically feel sluggish around 1 p.m. to 3 p.m. and again just after midnight, he says. Sitting around after a satisfying meal may make it that much easier to doze off.
Derek Chong, a professor of clinical neurology at Columbia, agrees that a person’s circadian rhythm can play a major role in meal-time lethargy. But he says food can push us over the edge and into a full-on siesta session. Food activates our parasympathetic nervous system, which regulates our resting and relaxation responses, as opposed to the sympathetic nervous system, which regulates our flight or fight responses, Dr. Chong says.
Tryptophan, of which turkey is a source, may predispose us to sleepiness, but we rarely consume enough for it to have a soporific effect, says Dr. Basner. Also, people usually ingest so many carbohydrates during the same meal, which are processed first by the brain for fuel. That makes it unlikely that tryptophan, which helps make proteins, would actually get to the brain, according to UT-Southwestern’s Ms. Sandon.
“You would have to eat the entire 20-pound turkey to get enough tryptophan to induce sleepiness,” she says.
By Christina Tsuei and Shirley S. Wangvia Wall Street Journal