Researchers from the University of Salzburg found that the brain continues to monitor the environment even when the person is asleep, especially during the first stage of sleep called the non-rapid eye movement (NREM) stage. The team claims that the brain is more active in detecting unfamiliar voices than familiar ones.
They published their study, titled "The Brain Selectively Tunes to Unfamiliar Voices During Sleep," in the journal JNeurosci on January 17, 2022.
The Brain Balances Sleep and Responding to Environmental Cues
Researchers observed 17 people while they were asleep in a lab for over two nights in the experiment. Their brain activities were monitored using an electroencephalography (EEG) machine, New Scientist reported.
Manuel Schabus, one of the study authors, said that the first night aimed to get participants comfortable with the new environment. Then on the second night, they played an audio recording of human speech on a loop from familiar and unfamiliar voices but at a volume that would not wake the participants.
In a news release via EurekAlert!, researchers said that they measured the brain activity of sleeping adults in response to familiar and unfamiliar voices in which they noticed that the latter elicited more K-complexes than the former. These are brain waves associated with sensory perturbances when the person is asleep.
Although familiar voices also trigger K-complexes, the team noted that unfamiliar voices cause great change in brain activity to its sensory processing. But when the voice became familiar after repeated trials, brain activity also occurred less often as the night went on. Researchers pointed out that the findings suggest K-complexes allow the brain to enter a "sentinel processing mode" in which the brain balances sleep and the ability to respond to environmental stimuli.
Schabus said that this suggests the brain could learn while asleep, but it does not mean that the brain can learn new words because it can cause more harm than help when the brain should be at rest.
An Evolutionary Trick to Respond to Potential Threats
Schabus said that the findings of the study make sense in terms of evolution. He explains that unfamiliar voices generate stronger brain activity because they should not be speaking to the person at night because it sets off an alarm. The New Scientist reported that this might explain why people often find it hard to sleep in unfamiliar environments, such as hotel rooms.
The brain continues to monitor the environment even when the eyes are shut, which is an evolutionary trick that humans developed, according to MailOnline, It goes back to the long process of human evolution that needs to quickly wake up in the face of potential danger coming from unfamiliar auditory stimuli.
Overall, the study suggests that unfamiliar voices prevent a restful night's sleep because of heightened brain activity. As Julie Darbyshire from the University of Oxford said, the findings show that unfamiliar voices can disturb sleeping people than a familiar one. For instance, patients are uncomfortable sleeping in hospitals because they are surrounded by equipment that produces unfamiliar and unpredictable sounds.
Read also: Mozart Effect: Scientists Explain Why This One Song Relaxes Brain Activity of Patients With Epilepsy
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