A team from the University of Salzburg has discovered that individuals’ brains still monitor their surroundings when they fall asleep. This is more evident during the non-rapid eye movement (non-REM) stage. Moreover, the researchers found that the brain was less alert to familiar voices than familiar ones.
The researchers gathered 17 people and asked them to sleep in the lab, where they observed them for more than two nights. They used an electroencephalography machine to monitor their brain activities.
According to a study author, Manuel Schabus, the team used the first night to make the participants comfortable in their new surroundings. On the second night, they played recordings of human voices while volunteers were asleep. They played them at a low volume so they wouldn’t wake up. The recording included both unfamiliar and familiar voices.
The team then looked at how the participants responded to the voices. They realized that they would elicit more K-complexes with unfamiliar voices. K-complexes are brain waves produced when people experience sensory disturbances as they sleep.
Unfamiliar voices caused more changes in K-complexes than familiar ones. However, the disparity would soon disappear as the volunteers heard the voices many times through the night, thus making them more familiar.
The team concluded that the K-complexes elicit a sentinel processing mode. In this state, the brain could be aware of stimuli in their surrounding while being in a state of sleep. Furthermore, this suggested that the brain could still learn when people slept. However, learning wasn’t complex as that would beat the purpose of rest.
The researchers also concluded that this was a necessary evolutionary adaptation as it allowed people to sense danger while they slept. This also allowed them to quickly get up and face an unknown danger that might occur while they were asleep. It could also serve as an explanation for why some people can’t sleep well in new environments.
A study author from the University of Oxford, Julie Darbyshire, adds that the study proves that unfamiliar voices could make people have a difficult time falling asleep in new places. For example, many patients report having a hard time sleeping in hospitals due to the sounds of unfamiliar machines. People who sleep in new hotel rooms also report the same thing.