During sleep, the brain understands if a voice is foreign and assesses its danger.

While we sleep, the brain continues to monitor our environment, including noises. He filters the voices of strangers, and if he listens to them several times, he realizes that they are not dangerous. That’s the interesting conclusion of a new study by researchers at the University of Salzburg, published in the Journal of Neuroscience.

The brain must always be alert, even during sleep

It is a balance between the brain’s attention to the outside world and the need for rest, which is obviously essential during sleep. The brain must rest while allowing for waking up at any time, if necessary.
One of the methods employed by the brain is to selectively recognize the voices of unknown human subjects by “filtering” them from those of known subjects, for example those of family members.


The researchers discovered this by performing experiments on different subjects during the sleep phase. While they slept, the researchers measured their brain activity in response to various voices from family members or people they didn’t know.
Compared to familiar voices, unfamiliar voices elicited more K-complexes.

Complexes K

K complexes are particular brain waves, perfectly detectable by electroencephalogram, which occur during phase 2 of NREM sleep (sleep with non-rapid eye movements). These brain waves have two functions in particular: they reduce wakefulness in the cortical area in the presence of external stimuli during sleep that do not represent a danger and they improve memory consolidation.
During the experiments, the voices of strangers caused large-scale changes related to sensory processing, changes that produced precisely more K-waves.

Sentinel Sleep Modes

Additionally, the researchers found that as time passed during the night and the unfamiliar voices repeated themselves, they became a little more “familiar” and the brains produced a lower and lower amount of K-complexes.
In practice, the brain continues to learn during sleep and is able to understand, even when we are completely unconscious, whether an external auditory stimulus can represent a danger or not. This is a “sentinel processing mode” implemented by the brain during sleep in order to retain the ability to respond, possibly upon awakening, to stimuli that may still be important.


  • The brain selectively attunes to unfamiliar voices during sleep – Journal of Neuroscience (DOI:/10.1523/JNEUROSCI.2524-20.2021)
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