This Is The Real Reason Why You Get Sleepy After a Thanksgiving Meal

Hint: Portion size doesn't always matter.

22 NOV 2017

You would think after stuffing ourselves with food, humans would have the energy to accomplish anything. On Thanksgiving, the reality is quite the opposite. After the huge holiday feast, many of us loosen our belts, stumble to the nearest couch and slip into a "food coma."


Despite the common nature of food comas – or what scientists like to call postprandial somnolence - many of us still don't understand why it happens.

Most people subscribe to the idea that food comas occur when blood leaves the brain and heads to the digestive tract. The theory follows the less blood oxygen in our brains, the less energy we have for anything other than a nap.

But as compelling as this theory sounds, it is actually a myth.  

While blood flow to the intestines and colon does increase after eating a large meal, that extra blood isn't coming from the brain. In fact, studies have shown there is no difference in the amount of blood flow to the brain after a big meal.

Plus, the idea that food comas affect everyone may not be true at all. Previous studies on postprandial somnolence have certainly shown a spike in sleepiness after eating, but they have also shown a lagging response or no effect at all. The results have led scientists to theorize that postprandial somnolence is extremely variable across the population. It may even be absent in particular individuals.


But for those of us that do experience postprandial somnolence, what the heck is going on?

When we eat a meal, our blood sugar spikes and, in response, our pancreas begins to produce a hormone called insulin. Insulin helps our bodies absorb the glucose and transports a variety of amino acids from the food we have eaten to our brain.  

One of these amino acids, called tryptophan, stimulates the release of serotonin and melatonin in our brains. Together, these two neuro hormones make you feel blissful and heavy-lidded.

And so the cycle goes: the more we eat, the more insulin our pancreas produces to catch up, leading tryptophan, serotonin and melatonin to flood our system. Eat, sleep and repeat.

The nature of this system has led many to believe the size of our meals is directly correlated to how sleepy we become after eating. But contrary to conventional wisdom, research suggests food comas have far more to do with what we eat than how much we eat.

That's because the amount of tryptophan varies across different food groups. For instance, foods with a high glycaemic index, like carbohydrates, tend to contain more tryptophan. 


Studies have shown eating food with a high glycaemic index makes humans fall asleep faster. On the other hand, meals that are high in protein contain amino acids that are far more stimulating. In combination, high and low glycaemic foods keep you at the perfect level of wakefulness.

It gets more complicated.

At the same time our bodies are producing hormones that regulate sleep, they are also inhibiting hormones that regulate wakefulness.

In our hypothalamus, there is a group of neurons that promote wakefulness by releasing a protein called orexin. But when you eat a normal meal, the production of orexin is switched off by tiny spikes of glucose.

The activity of these orexin neurons could explain why we get sleepy after a meal, and - on the flip side - why we often can't fall asleep when we are hungry. 

So, how can you avoid getting a food coma this Thanksgiving?

Watch your portion size and eat slowly. This allows the mechanism behind food comas time to balance out. By doing this, you ensure that your system isn't suddenly flooded by sleep-inducing hormones.     

If you can manage it, exercising after a meal can also help eliminate some of the blood sugar in your system, which allows your pancreas time to catch up.

But the best way to avoid a food coma is to ensure you eat a balanced meal made up of high and low glycaemic foods. After all, tryptophan has a negligible effect when it's combined with all the other amino acids, hormones and macronutrients found in a well-balanced diet.

Keep these tips in mind and you can go from postprandial somnolence to postprandial productivity in no time!