According to a new study, the human brain remains alert to unknown voices even during sleep in order to protect you against any possible threats.
With that being said, if you sleep with the TV on, you might want to change that habit to get the best sleep you can get at night.
The research was conducted by scientists from Austria who measured the brain activity of its adult subjects to both familiar and unfamiliar voices.
As it turns out, hearing unfamiliar voices while asleep causes the human brain to actually “tune in” to the sounds during NREM (non-rapid eye movement sleep), which is the very first stage of the sleeping cycle.
On the other hand, the researchers did not identify the same effect during REM, which is the deepest stage of sleep.
According to them, this is most likely because of microstructure changes in the brain.
Basically, the human brain continues to monitor everything happening around us even while our eyes are closed, managing to perfectly balance the need to wake up at any moment in the case of danger with the need to protect sleep.
The experts explain that one way the brain is able to do this is by responding, selectively, more to unfamiliar voices than to familiar ones.
Of course, this makes a lot of sense as the point is to alert you of any outside danger, an evolutionary trait that goes way back to the long process of human evolution.
Less familiar, auditory cues are obviously more likely to wake you up or at least disturb your sleep to a certain extent than hearing family members chatting quietly outside your bedroom door, for instance.
With unfamiliar voices, your brain will remain on high alert, however.
The study was led by University of Salzburg researchers and published in the journal JNeurosci.
In the paper, the team explains that “Our findings highlight discrepancies in brain responses to auditory stimuli based on their relevance to the sleeper. Results suggest that the unfamiliarity of voice is a strong promoter of brain responses during NREM sleep.”
The research involved 17 volunteers, 14 out of which were female, with an average age of 22.
None of the volunteers reported any sleep disorders before joining the study and were all fitted with polysomnography equipment throughout the night while sleeping.
What polysomnography does is measure one’s brain waves but also their respiration, heart activity, muscle tension, movements and more, keeping tabs on all changes as one goes through all the sleep stages.
Prior to the start of the study, the participants were told to follow a regular sleep and wake cycle involving about 8 hours of sleep per night for at least 4 days.
While they slept, auditory stimuli were played over loudspeakers including their first name and two other, unfamiliar first names, which were uttered by either a familiar voice or one of a stranger.
This is how the team was able to conclude that unfamiliar voices cause more K-complexes to happen, a sort of brain wave that is linked to sensory perturbances while sleeping.
And, as you may have already guessed, the familiar voices elicited K-complexes in comparison.
Of course, familiar voices can trigger K-complexes too but the ones elicited by unfamiliar voices also come with some large-scale changes in one’s brain activity that is linked to sensory processing.
But, as the night went on, the brain’s responses to the unfamiliar voice happened less and less as the voice started to be recognized, becoming more familiar in the end.
This suggests that even during sleep, one’s brain is still able to learn.
The scientists state that “It might be that our sleeping brain learns, through repeated processing, that an initially unfamiliar stimulus poses no threat to the sleeper and so, it decreases its response to it. Conversely, in a safe sleep environment, our brain might also be ‘expecting’ to hear familiar voices and inhibits any response to such stimuli in order to preserve sleep.”
Aside from K-complexes, the auditory stimuli during NREM sleep also managed to increase the numbers of “micro-arousals” and “spindles” in the brain.
As for what those are, study author Ameen Mohamed shared via MailOnline that “Spindles are faster brain waves appearing during NREM sleep and are linked to memory consolidation. Micro-arousals are periods in sleep during which the EEG signal shifts from slow and synchronised activity of sleep to faster, wake-like activity. By definition, they last from 3 seconds to 15 seconds; if they’re longer they are considered awakenings. They appear in all sleep stages.”
On the other hand, the scientists discovered zero differences between the number of triggered K-complexes, micro-arousals or spindles between the subject’s own first name and the name of strangers.
This part of the research is, in fact, quite interesting due to the fact that an older study from 1999 conducted by a French team, showed that one’s own first name could trigger a more powerful brain response than other names while sleeping.