Sleep: what is this surprising capacity that our brain has when we are asleep?

Even when we sleep deeply, our brain continues to monitor the environment, like a sentinel. It is thus able to distinguish between voices that are familiar to us and those that are not.

We know that, to help us fall into sleep , the brain is able to deactivate certain mechanisms and to put all its awakening systems in "silence" mode. But it does not fall asleep completely, and even when one is asleep it continues to monitor the environment, balancing the need to protect sleep against the possible need to wake up.

According to a study from the University of Salzburg, Austria, to achieve this sleep-wake balance, the brain has developed the ability to differentiate between familiar voices and unfamiliar ones, including when one is deeply asleep. It is thus able to respond to auditory stimuli according to their relevance to the sleeper .

For this study, the Austrian researchers measured the brain activity of about twenty sleeping adults, in response to familiar or unfamiliar voices. These voices pronounced their first name as well as two unknown first names while a polysomnography recorded the activity of the brain. Unknown voices thus elicited more K-complexes (a type of brain wave linked to sensory disturbances during sleep) and micro-arousals, compared to familiar voices. And while familiar voices can also trigger K-complexes, only waves triggered by unfamiliar voices were accompanied by large-scale changes in sleeper brain activity.

"These results suggest that K-complexes allow the brain to enter a 'sentinel processing mode', where the brain remains asleep but retains the ability to respond to relevant stimuli," the authors point out.