A Christmas day of indulgence – why do we get so tired?

Many of us have heard that drinking a glass of warm milk before bed will help sooth one to sleep. This common myth is based on the fact that foods containing the amino acid tryptophan can cause tiredness.


Image: Pixabay

This is a main candidate for why the family wide snooze after Christmas dinner is an annual tradition. Our favourite Christmas roast of turkey with all the trimmings is rich in tryptophan, the large and rather rare amino acid that can cause drowsiness. However, it cannot do this alone as it struggles to pass through the blood-brain barrier due to competition with other amino acids.

To cause its sleepy effects, tryptophan needs a transporter to carry it across into the brain. This transporter comes in the form of a small dose of carbohydrates (< 30g). Once inside the brain, tryptophan is involved in serotonin synthesis which is eventually converted into melatonin. This chemical is a main candidate for inducing sleep. The turkey you eat at Christmas increases the amount of tryptophan in your bloodstream but it’s that carb encrusted mince pie that helps tryptophan along into the brain and sends you into a dozy state. The other factor that contributes to tiredness at Christmas is the sheer amount of food that many people indulge in over the festive period – it is Christmas after all.

The blood providing oxygen to the brain gets partially redirected to the digestive system. The redirected oxygen is needed for digestion as muscles around your stomach and digestive tract contract and relax in a movement called peristalsis. Here the blood also receives glucose from digested food to carry around the body for respiration further afield. You may yawn in an effort to increase the oxygen to the brain that is lost to the digestive process yet that sleepy feeling persists, at least for a few hours.

But it’s not all ‘a second on the lips, a lifetime on the hips’. If you were revising over the Christmas period, a lot of the glucose from those Quality Streets went into brain function. The brain uses 20 per cent of our resting basal metabolic rate when we are doing nothing, about 300kcal. This is the same amount of energy used as swimming backstroke for 30 minutes!

With a Quality Street being 44kcal, and more glucose needed during stressful activities such as making a mind map and maintaining uninterrupted concentration in exams, it’s worth treating yourself to those extra sweets at the end of the day.

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