New research reveals that human brains pay attention to unidentified voices in sleep to remain alert to possible threats.
Austrian researchers measured brain activity in sleeping adults when they heard familiar or unfamiliar voices.
Unusual voices during sleep caused the brain’s ability to “tune in” during non-rapid sleep movement (NREM), which is the first stage.
They claim that researchers did not observe the effect in REM sleep (the deepest stage of sleeping) and this is likely because the brain’s micro-structure changed.
Even though we are asleep, our brain monitors what’s going on around us. It balances the need to sleep well with the need of waking up.
According to experts, one way it does this is to selectively respond to familiar voices rather than to unfamiliar ones.
This could possibly be due to our long evolutionary process and our instinct to rapidly wake up in case of danger.
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.
During sleep, the brain listens to strange voices. According to experts, this allows the brain (stock image), to adjust sleep to respond to environmental cues.
The study has been led by researchers at the University of Salzburg and published today in the journal JNeurosci.
The team states in their paper that their findings “underline discrepancies between brain responses to auditory stimuli, based on their relevance for the sleeper.”
“Results indicate that brain response to NREM sleep is strongly influenced by the familiarity of voice,”
Researchers recruited 17 female volunteers for the study (14 male) and an average age 22.
The volunteers, all of whom had no reported sleep disorders, were fitted with polysomnography equipment during a full night’s sleep.
The sleep stage changes can be measured using polysomnography. It measures brain waves and respiration as well as muscle tension, movement and heart activity.
Before the start of the experiment, participants were advised to maintain a regular sleep/wake cycle – around eight hours of sleep – for at least four days.
Volunteers were asked to keep a steady sleep/wake rhythm (each night around 8h) during the experimental period. This should be maintained for at most four days. They then spent the next two nights in lab. The first night they fell asleep while having polysomnography data (PSG), recorded, but not hearing any auditory stimulation. The second night, PSG data were recorded and auditory stimulation was provided by loudspeakers throughout the night. Both nights saw participants tested on their wakefulness prior to and following sleep.
While they slept they received auditory stiumuli through loudspeakers. They heard their first and second names. One was familiar (such as from a parent), the other unknown (a stranger).
Researchers found that unfamiliar voices elicited more K-complexes, a type of brain wave linked to sensory perturbances during sleep, compared to familiar voices.
Although familiar voices may trigger K-complexes as well, it was only the triggering of unfamiliar voices that caused large-scale brain activity changes related to sensory processing.
However, the brain responded less to the unknown voice as the night progressed, and the voice became familiarer, which suggests the brain might still be capable of learning during sleep.
These findings suggest that K-complexes enable the brain to enter the’sentinel process mode’. This allows the brain to remain asleep while still responding to appropriate stimuli.
According to experts, “It could be that the subconscious learns that an initial unfamiliar stimulus is not a threat and that consequently, it decreases the response.”
“Inversely,” the brain could be “expecting” to hear familiar voices in order to maintain sleep. In this case, it will continue to inhibit any such stimuli.
This graph illustrates the variation in micro-arousals as well as triggered K-complexes. Left, the difference between unfamiliar voice (UFV) and familiar voice (FV) in the number of triggered K-complexes was significant from 100ms to 800ms. In fact, there was a significant difference in the micro-arousals between FVs versus UFVs for periods of 200ms to 400ms and 500ms respectively.
Aside from K-complexes presenting auditory stimulations during NREM sleep increased both the number of brain’spindles and micro-arousals.
‘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.
“Micro-arousals” are times in which EEG signals shift from slow, synchronized activity to wakelike activity.
They last between three and fifteen seconds, according to the definition. If they go on for longer than that they’re considered awakenings. They are present in all stages of sleep.
Research showed no differences in trigger K-complexes or spindles between subject names.
This is interesting because previous research – including one 1999 study by a French team – has demonstrated that the subject’s own name evokes stronger brain responses than other names during sleep.
THE FOUR STAGES of Sleep
Different steps in the nighttime sleep cycle. Dreaming is most common in REM (referenced in red), but some dreams can occur in nonREM sleep.
There are four main stages to sleep. This is also known as NREM or non rapid eye movement.
This is also known as rapid-eye movement (REM) sleep.
A normal night of sleep is divided into the following stages.
Stage 1: In the first five minutes or so after dropping off we are not deeply asleep.
While we are aware of the surroundings, our muscles relax and our heartbeat slows. Brainwave patterns known as theta waves become more irregular, but faster.
Even though we’re asleep at Stage 1, you may feel like we didn’t sleep.
Our bodies transition into stage 2 after about five minutes.
Stage 2: This is when we have drifted into sleep, and if awakened would know you we been asleep. The process of waking up is relatively simple.
This stage is identified with short bursts in electrical activity, known as spindles. Larger waves, known as K complexes, indicate that the brain remains aware of its surroundings before it switches off to the sub-conscious.
The heartbeat, breathing and relaxation are slow.
The body’s temperature falls and the eye movements cease.
The brain wave activity is slowing but marked by short bursts electrical activity.
3. Stage: Stage 3 non-REM sleep is the period of deep sleep that we need to feel refreshed in the morning.
It happens more often in the first part of the night.
Sleep causes our heartbeat and breathing to slow, and the brain waves get even slower.
People may have difficulty awakening us because our muscles relax.
The body restores muscles and tissues, increases growth and develops immunity, builds energy, and boosts development.
Hypnagogia is a state of transition between wakefulness or sleep that can be referred to as NREM Stages one and three.
The mental phenomena that occur during hypnagogia are lucid thought and lucid dreaming.
The REM Sleep: REM sleep first occurs about 90 minutes after falling asleep.
Behind closed eyes, our eyes are able to move quickly from one side to the other.
Mixed frequency brainwave activity is closer to what you see in wakefulness.
The speed and irregularity of our breathing increases, as well as the heart rate and blood pressure.
While most dreaming happens in REM, there are some instances when it can happen in non-REM.
Temporarily paralysed leg and arm muscles, this prevents us dreaming.
Our time spent in REM sleep decreases as we age.
It is likely that memory consolidation will require both non-REM as well as REM sleep.
Source: US National Institutes of Health