Why Do You Get Tired After You Eat?

Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

The hours after a holiday meal are usually accompanied by extreme tiredness and the urge to take a nap. Because we eat food for fuel, this seems the opposite of what we might expect. So it's time to uncover the truth: why do we get tired after we eat?

Eating is Hard Work

We've all experienced it: we just ate a little too much for lunch, and now it's 2pm and we can hardly concentrate. Our entire bodies are telling us to go and take a nap. Why do we feel so tired during the mid afternoon? It may seem troublesome enough on a normal day, but it's amplified even more on holidays or at celebrations when we eat inordinate amounts of food. What is the cause of this change?

Providing energy is the reason we eat in the first place, so shouldn't eating increase our energy? In this lesson we are going to take a closer look at why the food crash happens.

Energy Levels

When we eat food, our bodies break that food down into a simple energy form that we can use to move, think, and otherwise operate our bodies. That form of energy is glucose, and it's the simplest type of sugar. When we eat, we are providing a body with glucose.

However, contrary to popular belief, when we eat food our increase in blood sugar doesn't affect our energy levels (that is, unless we were previously dangerously low on energy). In fact, our bodies work hard to keep our blood sugar levels from spiking to the point that it would impact us negatively, and keeps sugar levels in the brain relatively constant. When that doesn't happen it's a sign that something is wrong - it indicates that we have diabetes.

In fact, when scientists have studied the idea of a sugar high - specifically the hyperactivity often seen in kids after eating sugar - they found that it didn't exist. Although parents swore that they observed it, it turned out to be psychological. When the kids were randomized, or when the parents were lied to about which kids had sugar, the parents saw what they wanted to see.

So if a sugar high is fictional, what about a sugar crash?

Hormones and Tiredness

We've all experienced the intense tiredness you feel after a gigantic meal. Is this like the sugar high? Is it just anecdotal evidence with no validity?

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