I'm off for a nap

If you love a nap after Sunday lunch then scientists have come up with the perfect excuse - you are programmed to nod off by nature.

The discovery heralds the dawn of a new age of guilt-free afternoon snoozing for those who have enjoyed a good roast and perhaps a couple of pints.

It was already known the body has a mechanism to make us more alert when we need fuel. But know scientists have worked out why we get sleepy after a meal.

Glucose from the food inhibits nerve cells in the brain which send out signals to keep us alert and awake.

Tests on lab mice at Manchester University revealed that eating affects how these cells produce proteins called orexins which tell us we need to be alert.

Team leader Dr Denis Burdakov said: "This previously unknown mechanism detects minute changes in glucose levels such as those that occur between meals.

"This may well provide an explanation for after-meal tiredness and why it is difficult to sleep when hungry."

Other factors may be at work as well. All humans have a natural dip in alertness in the afternoon regardless of whether we eat.

The study is in the latest edition of the science journal Neuron.