THAT'S THE WHY:Here's a glimpse into the future: it's next Saturday evening, there are bits of wrapping paper all over the living room, your belly is full of Christmas dinner and you're settling down in front of the fire to watch the family movie on TV.

Within minutes you’re asleep, but why is it that Christmas evening is so slumber-inducing?

One theory that’s often bandied about is that it’s because turkey meat – which will turn up on many plates this weekend – naturally contains a biochemical called tryptophan.

We can’t make this amino acid ourselves, so we take it in the diet, and it’s an important component of serotonin and melatonin in the body, which help regulate mood and sleepiness.

So does eating turkey make you doze off? Actually, it’s probably not fair to place the blame on the bird alone. While it does contain tryptophan, its levels are not spectacularly high.

The apparently soporific effects of Christmas dinner might also be down to its burden of carbohydrates.

A high-glycaemic- index meal can boost levels of tryptophan in the blood and shorten the time to sleep onset, according to an Australian study published in the American Journal of Clinical Nutrition in 2007.

So on Christmas evening after the feast of turkey, spuds and plum pudding, add in the few drinks that might go with it and the feeling of (hopefully) being relaxed at home, and you’re a sitting target for those ZZZs . . .