Contrary to popular belief, the brain is active in sleep and can perform tasks, according to a new study.

The sleeping brain continues to pick external stimuli and categorises them, according to the study published in Current Biology.

Eighteen men and women were first given a set of words to classify as animal or object using the left or right hand when awake. They were then allowed to doze and given another set of words to classify.

While the hands did not move, the researchers registered brain activity directing the left or right hand to move.

They were next subjected to a set of real and pseudo words.

It was seen that the sounds were even then rightly categorised. The only difference was that the sleeping brain took a few seconds longer to process the information.

Lead author Sid Kouider, a cognitive neuroscientist at the École Normale Supérieure in France, told the Christian Science Monitor that the brain should, in theory, be able to do any task that we can accomplish while on "auto-pilot".

"Any procedure that can be simplified enough to become automatic, in principle should remain active while you're asleep."