Brains “Tune” to Unfamiliar Voices During Sleep

When humans sleep, their brains tune in to unfamiliar voices, according to new research. Such behavior is not well understood, but these new results seem to indicate that the brain balances the need to sleep with the need to wake up based on external sensory information.


Sleep is key to life. The Debrief has previously reported how vital sleep is for preparing us for the future, aiding creativity, interpreting dreams, and one day maybe even deep space travel.

Now, researchers are trying to understand how the sleeping brain filters and prioritizes sensory information, specifically auditory information, to determine if they should remain asleep or wake up.


According to a press release announcing the findings, researchers from the University of Salzburg looked at the brain activity of sleeping adults in response to both familiar and unfamiliar voices. “Unfamiliar voices elicited more K-complexes, a type of brain wave linked to sensory perturbances during sleep, compared to familiar voices.”

The researchers noted that familiar voices can also trigger these same K-complexes, but only the unfamiliar voices triggered complexes that were “accompanied by large-scale changes in brain activity linked to sensory processing.”

“Previous research has shown that sensory processing continues during sleep,” the paper published in the journal Jneurosci explains. “Here, we studied the capacity of the sleeping brain to extract and process relevant sensory information.

The research team enlisted 17 subjects to conduct the study, 14 of which were female. Once the subjects were asleep, specifically in non-rapid eye movement (NREM) sleep, the researchers enlisted someone with a voice each subject knew (a familiar voice, or FV) and someone with a voice each sleeper did not know (an unfamiliar voice, or UFV) and asked each one to speak the sleeping subjects’ names.

Each sleeper’s brain activity was recorded during the test using polysomnography, which can detect certain types of electrical activity in a sleeping brain. When the UFV and FV voices spoke the patients’ names, the readings indicated the brain was hearing them both. However, when the UFV spoke the sleepers’ names, a full range of responses indicated that the brain was actually analyzing and processing the speech, responses that were absent when the FV spoke the sleepers’ names.

“Our findings highlight discrepancies in brain responses to auditory stimuli based on their relevance to the sleeper,” the study explains. “We argue that such content-specific, dynamic reactivity to external sensory information enables the brain to enter a ‘sentinel processing mode’ in which it engages in the important internal processes that are ongoing during sleep while still maintaining the ability to process vital external sensory information.”

In essence, the sleeping brain is still “listening” to its environment, but this proposed ‘sentinel processing mode’ allows it to determine if that signal is worth waking up for, or if continued sleep is the higher priority.


Anyone who has tried to wake someone who is deep asleep may not be that surprised by these findings. However, the results are significant for folks who study sleep and how the brain operates while our bodies rest.

No follow-up study has was announced for the research. Still, the role of sleep is so critical to overall health that any more profound understanding of what can interrupt it, in this case an unfamiliar voice, is critical for future sleep studies.

Follow and connect with author Christopher Plain on Twitter: @plain_fiction