Even in deep sleep, our brain warns of danger

(ORDO NEWS) — Even when we fall asleep, our brain continues to work to keep us alive. It monitors the heartbeat and breathing, erases the garbage accumulated during the day, sorts and stores our memories. Now it appears to be doing all this and more while monitoring our surroundings for danger from strangers, a new study says.

“Unfamiliar voices shouldn’t talk to you at night – it’s alarming,” cognitive neuroscientist Manuel Chabus of the University of Salzburg told New Scientist.

Shabus and colleagues observed this alarm signal in 17 volunteers. After spending a night in the sleep lab, the volunteers underwent polysomnography, which recorded their brain waves, oxygen levels, heart and breathing rates, and movement.

“We presented the participants with audio recordings of their own names and two unfamiliar names. These names were pronounced either in a familiar or unfamiliar voice,” the first author of the study, cognitive neuroscientist Mohamed Amin, explained on Twitter.

Participants who were listened to softly reproduced unfamiliar voices reacted more actively than those who did not listen to them. These responses included microarousals – short bursts of brain activity, similar to awakening, that last only a few seconds. The function of microarousals is not yet fully understood.

Although both familiar and unfamiliar voices elicited brain waves called K-complexes, only those who heard unfamiliar voices experienced more significant changes in brain activity associated with sensory processing. It is believed that K-complexes keep you from waking up in response to harmless stimuli.

“K-complexes may be a key mechanism in determining how we sleep by helping the brain decide whether we should fall asleep or wake up,” Shabus told Inside Science. “It’s a pretty clever mechanism that allows you to filter out what matters or not, and when it matters, it starts a chain of processes to help process that information without having to wake up and disturb your sleep.”

Taken together, these results suggest that “the sleeping brain retrieves relevant sensory information for further processing,” Amid said.

This complements previous research showing that sensory processing of the environment continues even in unconsciousness, with the brain going into “watch mode” to perform this processing.

“Our results show that unfamiliar voices are more meaningful – or, from an evolutionary standpoint, potentially more threatening – and therefore more arousing to the sleeper than familiar voices,” the group writes in their paper.

However, the researchers did not rule out that this more excited reaction is not because the new voices are getting more attention in general, rather than being perceived as a possible threat.

However, while the response to familiar voices did not change after repeated sleep exposure, the brain’s response to unfamiliar voices did. This suggests that during sleep, the brain not only processed new information, but also learned from it, perhaps deciding that unfamiliar but repetitive noise was not a threat, dulling the future reaction to it.

These findings may help explain why we find it difficult to fall asleep in new surroundings – it takes our brains time to sort through all the unfamiliar sounds and decide we’re really safe in order to remain relaxed in ignorance.

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