Our brain pays attention to unfamiliar voices during sleep, study finds - Daily Mail

The human brain pays attention to unfamiliar voices during sleep to stay alert to potential threats, a new study reveals. .

Hearing unfamiliar voices when asleep caused the human brain to 'tune in' during non-rapid eye movement sleep (NREM), the first stage of sleep.

However, researchers didn't see the effect during REM, the deepest stage of sleep, likely due to micro-structure changes in the brain, they say. .

Even though our eyes are shut off from what's around us, the brain continues to monitor the environment as we sleep, balancing the need to protect sleep with the need to wake up. .

Overall, the study suggests unfamiliar voices – like those coming from a TV – prevents a restful night's sleep because the brain is on higher alert. .

The brain pays attention to unfamiliar voices during sleep.

'Results suggest that the unfamiliarity of voice is a strong promoter of brain responses during NREM sleep.' .

Polysomnography measures brain waves, respiration, muscle tension, movements, heart activity and more, as they advanced through the different sleep stages. .

Researchers found that unfamiliar voices elicited more K-complexes, a type of brain wave linked to sensory perturbances during sleep, compared to familiar voices

While familiar voices can also trigger K-complexes, only those triggered by unfamiliar voices were found to be accompanied by large-scale changes in brain activity linked to sensory processing, they found

However, brain responses to the unfamiliar voice occurred less often as the night went on and the voice became more familiar, indicating the brain may still be able to learn during sleep. 

'Conversely, in a safe sleep environment, the brain might be "expecting" to hear familiar voices and consistently inhibits any response to such stimuli to preserve sleep.'  

As well as K-complexes, presenting auditory stimuli during NREM sleep increased the number of 'spindles' and 'micro-arousals' in the brain

'Spindles are faster brain waves that appear during NREM sleep and are linked to memory consolidation,' study author Ameen Mohamed at University of Salzburg told MailOnline. 

Pictured, different steps of the night sleep cycle. Most dreaming occurs during REM sleep (marked in red) although some can also occur in non-REM sleep

This stage is identified by short bursts of electrical activity in the brain known as spindles, and larger waves known as K-complexes, which indicate that the brain is still aware of what is going on around it before turning off to a sub-conscious level.  

Most dreaming occurs during REM sleep, although some can also occur in non-REM sleep.