Why does Christmas dinner make us sleepy?

I hope you all had a lovely Christmas, and are looking forward to the New Year. At about 5pm on Christmas day I looked round our living room to see my sister, dad and grandparents all fast asleep – the annual post Christmas lunch nap. Something which caused me to ponder why, so I thought I’d do a bit of research and share it with you in a quick blog post.

Initially, the only answer I found was that large meals obviously take a long time to digest, thus blood is diverted away from other body areas to help digest the food at a faster rate. However, after a bit of digging I found some interesting websites and articles which gave more in-depth alternatives. The conclusion I came to is as follows…

Eating triggers the PNS  (parasympathetic nervous system) responsible for preparing the body for rest and increasing the activity of the digestive system. The PNS is part of the autonomic nervous system (thank you 3rd Form biology!) which is responsible for involuntary actions. Fundamentally, the actions of the PNS trigger hormones and neurotransmitters which are what make us feel sleepy.

An old New Scientists article says that ‘high blood glucose levels, similar to those after eating a big meal, can switch off the brain cells that normally keep us awake and alert.’ Additionally to this, high blood glucose levels cause the PNS to stimulate the pancreas to produce insulin – converting these sugars to be stored.  This increased level of insulin consequently stimulates the action of tryptophan, an essential amino acid within the brain. In turn, when the tryptophan is in the brain, it leads to an increased level of serotonin – the universally known ‘happy hormone’. Serotonin is an neurotransmitter which passes electrical signals between connecting neurones, and has many functions, including controlling mood and lethargy. Around 90% of serotonin within the body is found in the abdomen, and is responsible for regulating intestinal movements. However, the remaining 10% is found in the brain.

In short, high blood glucose levels trigger the production of insulin. This stimulates the action of tryptophan in the brain, consequently triggering an increase in the levels of serotonin within the body.

Therefore, it is the increased level of serotonin, responsible for mood and ‘sleepiness’ which makes you feel like all you can do is nap after Christmas dinner!


  1. https://www.newscientist.com/article/dn9272-why-we-need-a-siesta-after-dinner/
  2. https://www.howitworksdaily.com/why-do-we-feel-sleepy-after-eating/
  3. https://www.quora.com/Why-do-I-experience-tiredness-and-fatigue-after-eating

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