Are 'food comas' real or a figment of your digestion?

(CNN) You've just finished a big meal, and you're stuffed. You're thinking of taking a walk or even a stretch before clearing the table, but the thought of just getting out of your chair seems like a challenge.

The idea of taking a snooze on the couch seems more and more appealing.

Sounds familiar? There's a name for it: It's called postprandial somnolence -- commonly referred to as "food coma." The phenomenon refers to the tired, sleepy feeling that many of us experience after eating a big meal. And the causes are based on different theories, some more plausible than others.

Blood flow shifts

According to David Levitsky, professor of nutrition and psychology at Cornell University, the most likely explanation for food comas has to do with changes in circulation. When food enters your stomach and activates the gastrointestinal tract, "blood flow shifts from the muscles and brain into the stomach and intestines," he explained. "And 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."

These blood shifts occur because eating a meal activates our parasympathetic nervous system, the part of our nervous system that triggers digestion and absorption of food. It's been dubbed the "rest and digest system," as its aim is to conserve energy as it slows heart rate and increases intestinal activity, preparing the body to assimilate a meal's nutrients.

On the flip side, our sympathetic nervous system takes on the opposite role: It's triggered in response to a threat or danger and induces a "fight or flight" response, resulting in increased heart rate and increased blood flow to the brain.

Bigger meals have more effects

You're not at risk of falling into a food coma if you're just having a few bites of a snack, though. "It's got to be a large meal," Levitsky said. "The parasympathetic nervous system is activated when you eat, but (the extent to which it induces sleepiness) depends on the magnitude of the meal."

How does it work? It has to do with gastric distention, or the stretching of the stomach after we eat a large amount of food. The bigger the meal, the more distention and the greater the effects.

"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," explained Dr. William Orr, a clinical professor of medicine at the University of Oklahoma Health Sciences Center who has published studies on the topic.

Also, a liquid meal seems to make us less tired than a solid meal. "Our study found that in contrast to a liquid meal, a solid meal creates more sleepiness when compared to an equivalent volume of water," Orr said.

Part of the reason is that liquids and solids stimulate different parts of the stomach and have different feedback stimuli to the brain, according to Orr. Liquids are primarily handled in the upper part of the stomach, known as the fundus, whereas solids are primarily processed in the antrum, or the lower part. It's more likely that the antrum has neural connections in the brain that are more likely to induce sleepiness, he said.

Hormone theories: CCK, serotonin and tryptophan

CCK is an intestinal hormone that rises in the blood after you eat, a fact well-known and documented in rats, according to Levitsky. And in rats, injections of CCK at very high doses put them to sleep. "But if you infuse CCK through a catheter until it reaches the concentrations normally seen after eating a meal, rats don't necessarily get sleepy," Levitsky said. The bottom line: CCK may not play a meaningful role in post-meal fatigue after all.

A similar case can be made for serotonin, a neurotransmitter that is produced from the amino acid tryptophan and is responsible for feelings of relaxation and sleepiness. It's true that after we eat, the amount of tryptophan in the blood increases, especially if we eat a lot of carbohydrates -- and this would theoretically lead to greater production of serotonin in the brain. However, the amount of tryptophan that increases after we eat is not great enough to raise brain serotonin levels.

"If you infuse serotonin directly into the brain, it will make you go to sleep, but like CCK, the concentration necessary to produce this effect is supraphysiological," Levitsky said. "The direction of the theory is correct, but the quantification is not."

Protein, fat and circadian rhythms

Protein delays gastric dumping, which means food, and its surrounding supply of blood, tends to stay in the stomach longer.

"It is theoretically possible that after eating a large (protein-rich) meal, you may feel more tired," Levitsky said. Indeed, a recent study involving fruit flies found that the more protein the flies consumed, the longer they slept. A similar case can be made for fat, which also takes longer to digest -- though when eaten with carbs, small amounts of protein or fat can help stave off sleepiness by slowing the rise in blood sugar after a meal, which can help prevent blood sugar spikes and crashes.

Circadian rhythms contribute to food comas, too. In our bodies, there is a normal decrease in arousal that occurs in the early to mid-afternoon, a biological phenomenon that contributes to sleepiness and is compounded by eating a meal.

"Around 1 or 1:30 p.m. is right about when that dip occurs, where we are a little more drowsy," Orr explained, though that time can vary based on when you wake up. "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."

There's also a dip in the evening -- due largely to the normal circadian decrease in body temperature that occurs at this time -- and a large dinner would compound the food coma effect. But for most, feeling sleepy when it's closer to bedtime serves us better than becoming drowsy earlier in the day. "In the mid-afternoon, you're occupied with the tasks of daily living, and getting very sleepy does not help!" Orr said.

Preventing a food coma

Though a food coma may seem inevitable at times, there are some things you can do that may help:

• Eat small meals. The bigger the meal, the greater the chance you'll be drowsy. "We have to consciously put small amounts of food on our plates," says Levitsky. At lunchtime, small portions are especially important, because the lunchtime dip in arousal compounds the effects. "If I want to avoid postprandial sleepiness, I will have a light lunch, because I know no matter what, at 1 or 2 p.m., I will be sleepy, even if I don't eat anything," Orr said.
• Have an early lunch. "If you eat at 1 p.m., that's right at the time of the endogenous circadian dip," Orr said. It's better to eat an earlier lunch about 11:45 a.m. rather than a later lunch at, say, 1 or 1:30, right when that dip occurs.
• Choose liquids over solids. That doesn't mean lunch has to be limited to smoothies, though occasionally they're fine as a mini meal. "If you have a salad or a bowl of soup as opposed to a hamburger -- something with more of a liquid consistency, with a higher water content -- that's better," Orr said.
• Opt for carbs that are low on the glycemic index over ones that are high. Low-GI carbs include whole wheat bread, oatmeal, beans, peas, most fruits and non-starchy vegetables. Limit white bread, white rice, bagels, pretzels and crackers.
• Grab a cup of coffee. "You can counter the effects of sleepiness after a meal by consuming caffeine," Levitsky said. A cup of coffee or cappuccino should suffice. Though caffeine can serve as a helpful stimulant, too much can lead to restlessness and can interfere with sleep later on.
• Skip the wine and martinis. "Alcohol is a sedative, so this just adds to the drowsiness," Orr explained. If you enjoy a drink with a meal, choose dinnertime over lunch, and limit yourself to one beverage.