If you ever feel tired and lazy after a large meal, preferring to take a nap instead of a walk, you're probably going through postprandial somnolence, or as it's more widely known, a food coma.
Scientists still don't know exactly what prompts this, but according to David Levitsky, a professor of nutrition and psychology at Cornell University, the most probable cause has to do with changes in blood circulation.
He explained that when you eat it triggers "blood flow shifts from the muscles and brain into the stomach and intestines."
As quoted by CNN, Levitsky continued, "When blood volume goes down in the brain, we get woozy and tired. It's why I have to make my lectures extremely exciting: They're right after lunch."
The more food you eat, the more likely you are to fall into a food coma. "It's got to be a large meal," Levitsky clarified.
"The parasympathetic nervous system is activated when you eat, but [the extent to which it induces sleepiness] depends on the magnitude of the meal."
The odds of getting a food coma also depend on the type of food consumed. For example, a liquid meal would make us less tired than a solid one.
Dr. William Orr, a clinical professor of medicine at the University of Oklahoma's Health Sciences Center, stated that "If you have a large meal, the [degree of] gastric distention and hormonal stimulation that occurs will make you sleepier than if you had a bowl of soup," and his "study found that in contrast to a liquid meal, a solid meal creates more sleepiness when compared to an equivalent volume of water."
According to Orr, this is because liquids and solids activate different parts of the stomach and stimulate different parts of the brain.
Liquids are processed in the upper section of the stomach, in the fundus, whereas solids are processed in the lower section, in the antrum.
It's more likely that the antrum has connections in the brain that are prone to induce sleepiness, he said.
It is also possible that protein is the biggest trigger of postprandial somnolence, as proteins delay gastric dumping, which causes food and its surrounding supply of blood to remain in the stomach for longer, causing you to feel tired and bloated.
"It is theoretically possible that after eating a large (protein-rich) meal, you may feel more tired," Levitsky said.
His claims are backed by a study in which fruit flies were given differing types of food, and it was found that the fruit flies that were given most protein were the ones that slept the longest.
Circadian rhythms may also play a part in postprandial somnolence. It is normal for there to be a dip in energy around noon, which would further be affected by meals.
While this exact time depends on what time you wake up, Orr explained that, "Around 1 p.m. or 1:30 p.m. is right about when that dip occurs, where we are a little more drowsy.
Even if you don't eat lunch, you would still get sleepy due to the circadian rhythm. But when we eat at this time, it's a double whammy."
If you want to avoid the dreaded food coma, nutritionists recommend skipping the alcohol, having an early lunch and eating smaller meals.