The human brain is aware of unknown voices during sleep to be aware of potential threats, a new study reveals.

Researchers in Austria measured the brain activity of sleeping adults in response to known and unknown voices.

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

But the researchers did not see the effect during REM, the deepest stage of sleep, probably due to microstructural changes in the brain, they say.

Although our eyes are closed by what is around us, the brain continues to monitor the surroundings while we sleep, balancing the need to protect sleep with the need to wake up.

One way to achieve this is by selectively responding to unknown voices rather than familiar ones, according to the experts.

This can go back to the long process of human evolution and the need to quickly wake up to potential danger, characterized by lesser known auditory signals.

Overall, the study suggests that unknown voices - such as those coming from a television - prevent a restful night's sleep because the brain is on higher alert.

The brain is aware of unknown voices during sleep. This ability allows the brain to balance sleep by responding to environmental signals, according to experts (stock image)

The study was led by researchers at the University of Salzburg and published today in the journal JNeurosci.

"Our results highlight inconsistencies in the brain's responses to auditory stimuli based on their relevance to the sleeper," the team says in their paper.

'Results suggest that unfamiliar voice is a strong promoter of brain reactions during NREM sleep.'

For the study, researchers recruited 17 volunteers (14 women) with an average age of 22 years.

The volunteers, all of whom had not reported sleep disorders, were provided with polysomnography equipment during a full night's sleep.

Polysomnography measures brain waves, respiration, muscle tension, movement, heart activity and more as they progressed through the various stages of sleep.

Prior to the start of the trial, participants were advised to maintain a regular sleep / wake cycle - about eight hours of sleep - for at least four days.

Prior to the experiments, volunteers were advised to maintain a regular sleep / wake cycle (approximately 8 hours of sleep) for at least four days. Then they spent two nights in the laboratory - the first they slept with polysomnography (PSG) data recorded, but they heard no auditory stimulation. For the second night, PSG data was recorded while auditory stimulation came from speakers through the night. In both nights, participants were tested during wakefulness before and after sleep

While asleep, they were presented with auditory ulcers via speakers with their own first name and two unknown first names spoken by either a known voice (such as a parent) or an unknown voice (a stranger).

Researchers found that unknown voices evoked more K complexes, a type of brain wave associated with sensory disturbances during sleep, compared to familiar voices.

While well-known voices can also trigger K-complexes, they found that only those triggered by unknown voices were accompanied by major changes in brain activity associated with sensory processing, they found.

However, brain reactions to the unknown voice occurred less frequently as the night went on and the voice became more familiar, indicating that the brain may still be able to learn during sleep.

These results suggest that K-complexes allow the brain to enter a 'sentry processing state' where the brain remains asleep but retains the ability to respond to relevant stimuli.

"It may be that the dormant brain, through repeated processing, learns that a initially unknown stimulus does not pose any immediate threat to the dormant and consequently diminishes its response to it," the experts say.

"Conversely, in a safe sleep environment, the brain can" expect "to hear familiar voices and consistently inhibits any response to such stimuli to preserve sleep."

The graph shows the difference in the triggered K-complexes and micro-arousals. To the left, the difference between unknown voice (UFV) and known voice (FV) in the number of triggered K complexes was significant from 100 ms to 800 ms. The difference in the number of micro-arousals between FVs and UFVs was significant in the periods from 200 to 400 ms and from 500 to 700 ms.

In addition to K-complexes, presentation of auditory stimuli during NREM sleep increased the number of 'spindles' and 'micro-arousals' in the brain.

"Spiders are faster brain waves that occur during NREM sleep and are associated with memory consolidation," study author Ameen Mohamed of the University of Salzburg told MailOnline.

Micro-arousal are periods of sleep in which the EEG signal changes from slow and synchronized sleep activity to faster, awake-like activity.

»Pr. by definition, they last from three seconds to 15 seconds; if they are longer, they are considered awakenings. They occur in all stages of sleep. '

However, researchers found no difference in the amount of triggered K-complexes, spindles or micro-arousals between the subject's own name and unknown names.

This is interesting because previous research - including a 1999 study by a French team - has shown that the subject's own name elicits stronger brain reactions than other names during sleep.

THE FOUR STAGES OF SLEEP

In the picture, different steps in the night sleep cycle. Most dreams occur during REM sleep (marked in red), although some may also occur in non-REM sleep

Sleep is generally separated into four phases. The first three of these are known as 'non-rapid eye movements' or NREM sleep.

The last stage is known as rapid eye movement or REM sleep.

A typical night's sleep goes back and forth between the stages.

Scene 1: For the first five minutes or so after delivery, we do not sleep deeply.

We are still aware of our surroundings, but our muscles begin to relax, the heartbeat slows down, and brain wave patterns, known as theta waves, become irregular but rapid.

Even though we are sleeping during step 1, we may wake up from it with a feeling that we are not sleeping at all.

After about five minutes, our bodies move into phase two.

Stage 2: That is when we have fallen asleep and if we woke up would know we were asleep. Waking up is still pretty easy.

This stage is identified by brief outbursts 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 it off to a subconscious level.

Heartbeat and breathing are slow and the muscles relax even more.

Our body temperature drops and eye movements stop.

Brainwave activity decreases, but is characterized by short bursts of electrical activity.

Stage 3: Step 3 non-REM sleep is the period of deep sleep that we need to feel fresh in the morning.

It occurs for extended periods during the first half of the night.

Our heartbeat and breathing slowly to their lowest levels during sleep and brain waves become even slower.

Our muscles are relaxed and it can be hard for people to wake us up.

The body repairs muscles and tissues, stimulates growth and development, boosts the immune system and builds energy for the next day.

Hypnagogy - the transition state between wakefulness and sleep - is associated with NREM stages one to three.

Mental phenomena during hypnagogy include clear thoughts, clear dreams, hallucinations, and sleep paralysis.

REM sleep: REM sleep does not occur until about 90 minutes after you fall asleep.

Our eyes move rapidly from side to side behind closed eyelids.

Mixed frequency brainwave activity becomes closer to that seen by wakefulness.

Our breathing becomes faster and irregular, and heart rate and blood pressure rise to almost awake levels.

Most dreams happen during REM sleep, although some can also occur in non-REM sleep.

Arm and leg muscles are temporarily paralyzed, which prevents us from living out our dreams.

As we get older, we spend less of our time in REM sleep.

Memory consolidation most likely requires both non-REM and REM sleep.

Source: US National Institutes of Health