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Some parts of the brain unconsciously continue to make classifications of the words, even in sleep, according to a new study.  

"We show that the sleeping brain can be far more 'active' in sleep than one would think," said one of the authors of the study, Sid Kouider of Ecole Normale Supérieure, in the 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."

The study extends earlier work on subliminal processing by showing that speech processing and other complex tasks "can be done not only without being aware of what you perceive, but [also] without being aware at all," the press release added.

Researchers recorded the EEG of human participants while they were awake and instructed to classify spoken words between animals or objects. Eventually, participants were put in the darkened to recline comfortably with eyes closed while the classification task was continued, as they drifted to sleep.

Researchers noted that, even when they were asleep, participants continued to respond accurately, although more slowly, even as they lay completely motionless and unaware.

"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," Kouider added.

The study has been published in the journal Current Biology.