The brain is still active while we are asleep, say scientists, who found people were able to classify words during their slumber.
Researchers from Cambridge and Paris introduced participants to a word test while awake and found they continued to respond correctly while asleep.
The sleeping brain can perform complex tasks, particularly if the task is automated, the study says.
Further research will now focus on how to take advantage of our sleeping time.
Writing in the journal Current Biology, the research team set out to study the brain's behaviour while awake and during sleep.
Using an electroencephalogram (EEG), they recorded the brain activity of participants while they were asked to classify spoken words as either animals or objects by pressing a button.
Participants were asked to press a button in their right hand for animals and in their left hand for objects.
This allowed researchers to track the responses and map each word category to a specific movement in the brain.
Then participants were asked to lie down in a darkened room with their eyes closed and continue the word classification task as they drifted off to sleep.
Once asleep, a new list of words was tested on participants to ensure that the brain had to work out the meaning of the words before classifying them using the buttons.
Their brain activity showed they continued to respond accurately, the researchers said, although it happened more slowly.
At the time, the participants were completely motionless and unaware.
Sid Kouider, from the Ecole Normale Superieure in Paris, said: "We show that the sleeping brain can be far more 'active' in sleep than one would think.
"This explains some everyday life experiences such as our sensitivity to our name in our sleep, or to the specific sound of our alarm clock, compared to equally loud but less relevant sounds."
He added that it was possible for people to perform calculations on simple equations while falling asleep and then continue to identify those calculations as right or wrong during a snooze.
Any task that could become automated could be maintained during sleep, he said. But tasks that cannot be automated would stop as sleep took over.
Their research could lead to further studies on the processing capacity of our sleeping brains, the study said.
"Research focusing on how to take advantage of our sleeping time must consider what is the associated cost, if any, and whether it is worth it," Mr Kouider said.