Do carbs make you sleepy?

Post-prandial somnolence, as the medical literature likes to call it (presumably in case Julius Caesar is reading) has many causes. Despite their reputation as a source of energy carbohydrates often get the blame. Justifiably, it turns out, because eating carbohydrates on their own can make you sleepy.

Carbohydrates, to be clear, are starches and sugars. Starch molecules are just long chains of sugars which are broken down by saliva, so by the time it reaches your blood stream carbohydrate means sugar and sugar largely means glucose.

Increased glucose in the blood stimulates insulin release. This in turn causes most amino acids — the building blocks of protein — to be sucked out of the blood by the body. One exceptions is tryptophan, which continues merrily circulating as you munch on your starchy or sugary food of choice.

Now the clever part. The rate at which amino acids cross from the blood to the brain is limited and when the other amino acids drop off tryptophan has a much easier time competing to cross over. Once in the brain it causes a surge of the neurotransmitter serotonin, of which it is the key ingredient. Serotonin is a busy molecule but one of its jobs is controlling sleepiness.

That’s how we think it happens. As you can see it’s a long and bumpy ride from cake and chips to sleepiness, which means it is easy to muck up the neat little picture above.

Let’s throw some protein into the mix. Now digested amino acids will flood into the bloodstream despite the insulin. The tryptophan has a harder time competing to get from the blood to the brain so the serotonin burst is smaller, the sleepiness less. The more protein added the blunter the effect.

So a balanced meal probably won’t put you up or down. But eat a meal of pure carbohydrate and the arms of Morpheus will soon beckon. If you’d rather Morpheus kept his arms to himself, just add protein.

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