It has been known that our brain never fully rests when we sleep, as it has to keep our organs working and keep track of our breathing. But it turns out this is not the full list of its "night duties".

To explore how our brain is monitoring our surroundings, Manuel Schabus from the University of Salzburg in Austria and his colleagues monitored 17 people in a sleep lab for two nights. While the participants became accustomed to their surroundings in the sleep lab on the first night, during the second night the scientific team played an audio recording of human speech on loop. The voices were either familiar to the participants or unfamiliar.
The voices would utter three first names, with the two common but unfamiliar, and the third belonging to the sleeper. When the sleepers heard unfamiliar voices, the scientists recorded an increase in K-complexes – a type of brainwave that is slow and isolated. K-complexes, according to Schabus, show "immediate response to a disturbance".
"This study shows that unfamiliar voices disturb sleeping people more than familiar ones", Julie Darbyshire from the University of Oxford said as cited by the New Scientist. "We see these effects when hospital patients find it very hard to sleep".
According to Schabus, evolutionarily, it makes sense that unfamiliar voices trigger stronger brain activity than those that we know. However, in the second half of the night the number of K-complexes evoked due to the unfamiliar sounds went down, the scientists found.

"It means we can learn something new in the near-unconscious state", Schabus said.
The findings, however, do not indicate that it is good for us to try and learn new words as we sleep, Schabus warned, referring to the common practice for learning new languages.