Falling asleep after eating your Sunday lunch is a natural reaction, according to new research by brain scientists.
It's not the comfort of the armchair, the warmth and softness of the cushions or the after-effects of roast beef washed down by a pint or two that induces sleep, the experts say.
Instead, tests on lab mice reveal it is due to glucose in food inhibiting the nerve cells that send out signals to keep the brain alert and awake.
Scientists already know humans have a built-in mechanism which means when the body needs fuel, the brain chemistry creates alertness.
And just as nerve cells keep us awake if we are hungry, the cells turn off after we eat - making us sleep.
Researchers at the University of Manchester identified the nerve cells in the brain that produce tiny proteins called orexins.
It was known these cells promote wakefulness by 'firing' signals to the brain to keep it alert.
Dr Denis Burdakov said: "We have identified the pore in the membrane of orexin-producing cells that is responsible for the inhibiting effect of glucose.
" This previously unknown mechanism is so sensitive it can detect minute changes in glucose levels - the type that occurs between meals for example."
The study is in the science journal Neuron.