Your brain doesn’t go off to sleep when you do. In fact, recent research has found that even in sleep the brain is relatively active and is able to recall simple words that you may have read before nodding off. An experiment, which shows that some parts of the brain behave the same whether asleep or awake, was recently demonstrated by scientists from Paris and the University of Cambridge. A report on the experiment was published in Cell Press journal Current Biology.

The experiment showed that when people were subjected to simple word classification tasks before sleeping, the brain continues to unconsciously make classifications even in sleep. This research is a step toward understanding the vast untapped potential of the human brain, say researchers.

"We show that the sleeping brain can be far more 'active' in sleep than one would think," said Sid Kouider of Ecole Normale Supérieure, Paris, in a press release. "Far from falling [into] a limbo when we fall asleep, parts of our brain can routinely process what is going on in our surroundings and apply a relevant scheme of response. 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."

In the experiment, participants were asked to classify simple words belonging to one of the two categories — animals or objects — by pressing a button with the right hand for animals and with the left for objects. Their EEG was also simultaneously recorded.

 The procedure allowed Kouider and his colleagues to compute lateralized response preparations — a neural marker of response selection and preparation — by mapping each word category to a specific plan for movement in the brain. Once the rule for classification had become automatic for the participants, they were placed in a darkened room and asked to continue the classification till they went off to sleep.

The results showed that once a rule had been established while awake, it was implemented even during sleep. Once asleep, the participants were further tested with a new set of words to make sure the brain registered the meaning of the word rather than a simpler pairing between stimulus and response.

The results again showed that the participants continued to respond accurately, albeit slower than when they were awake.

Earlier studies on subconscious speech processing and other complex tasks have shown that such tasks can be done without even being aware of them, meaning subconsciously.

Kouider suspects that such unconscious processing isn't limited by the complexity of the task, but by whether it can be made automatic or not. So according to him, if a person has been working on solving simple equations before going off to sleep, the brain will continue to assess whether the calculations are correct or not even in sleep.  The brain will continue to process any task that can be automated while tasks that can be automated will stop when sleep takes over, he says.

Source: Kouider, Andrillon T,  Barbosa L, et al. Inducing task-relevant responses to speech in the sleeping brain, Current Biology. 2014.