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The sleeping brain is more active if it hears unfamiliar voices rather than familiar ones. The finding suggests that we can process information about our environments even in the depths of sleep.

at the University of Salzburg in Austria and his colleagues monitored 17 people, with an average age of 23, in a sleep lab over two nights. Brain activity was monitored using an electroencephalography (EEG) machine.

“The first night was so that the subjects could get comfortable with their new environment,” says Schabus.

During the second night, while the participants were asleep, they played an audio recording of human speech on loop. The voice was either unfamiliar to the sleeper or belonged to a familiar person, such as a parent or a romantic partner.

In either case, the voice repeatedly uttered three first names: two random but common names and the name of the sleeper. The audio recordings were played for four 90-minute periods during the night. There was a 30-minute gap between each audio recording so that it would be easier for people to stay asleep.

The audio was played at a volume so as not to wake the participants up. “We adjusted the sound levels individually,” says Schabus.

The researchers found that unfamiliar voices generated more brain activity in the sleepers than familiar voices. In particular, they found an increase in the number of K-complexes – a type of brainwave that is slow and isolated – when the subjects heard unfamiliar voices.

“K-complexes are interesting because they show the immediate response to a disturbance,” says Schabus. That response is divided into two parts, he says: first, the brain processes the information, then it inhibits the information so it doesn’t wake up the sleeping individual.

If the participant’s brain activity suggested that they were on the verge of waking up, the researchers lowered the volume of the recordings to help them stay asleep.

Schabus says it makes sense evolutionarily why unfamiliar voices generate stronger brain activity than familiar ones. “Unfamiliar voices should not be speaking to you at night – it sets off an alarm,” he says.

The finding may be part of the reason why we sometimes struggle to sleep in new environments, such as hotel rooms, says Schabus.

“This study shows that unfamiliar voices disturb sleeping people more than familiar ones,” says at the University of Oxford. “We see these effects when hospital patients find it very hard to sleep.”

“Partly, this is because almost nothing in the environment is familiar. As well as unfamiliar voices, patients will also be surrounded by equipment with unfamiliar and unpredictable pings, bongs and beeps.”

Unfamiliar voices also triggered fewer K-complexes in the second half of the night compared with the first half. “It means we can learn something new in the near-unconscious state,” says Schabus.

But he notes that this doesn’t mean we can learn new words during sleep. “You need the night to sleep and rest and if you don’t sleep properly, it does more harm than good for learning,” he says.