Scientists have explained nighttime anxiety due to ordinary sounds
During the day, an incomprehensible sound behind the wall can easily be mistaken for the creaking of an old house or the noise of neighbors. But it's worth waking up in the middle of the night from the same rustle, as the imagination already draws an uninvited guest in the hallway. In the dark, even familiar sounds can seem suspicious, and sometimes a person hears things that didn't happen in the room at all. It's not just the fear of the dark: the brain is constantly trying to explain incoming signals and assess whether we are in danger. About why an ordinary creaking turns into a reason for panic at night and where the sounds that no one made come from — in the Izvestia article.
The brain doesn't shut down with us.
Sleep is not a complete disconnection of the brain from the outside world. Even when a person is asleep, the nervous system continues to process some of the external signals. This is necessary in order to notice important changes in the environment and, if necessary, to wake up. At the same time, the brain does not react to every sound in the same way: familiar and repetitive noises can go unnoticed, while a sharp or unusual signal can attract attention.
The authors of a study on sound processing during sleep, published in the journal Trends in Cognitive Sciences, described a kind of continuum between complete sensory isolation and maintaining communication with the environment. A sleeping person may not react to loud but insignificant sounds, while still being able to process important signals, such as their own name. According to scientists, sensory isolation can promote recovery processes and memory consolidation, while maintaining communication with the environment helps to detect potential threats.
This feature is confirmed by an experiment published in the journal Scientific Reports. Scientists have studied the reactions of sleeping people to various vocal signals, including screams. The study involved 23 volunteers, but the final analysis included data from 15 people. The participants' brain activity was recorded using electroencephalography. It turned out that screams caused more pronounced brain reactions even during slow-wave sleep. This is attributed to the acoustic roughness of sounds — the rapid fluctuations in volume characteristic of screams and alarms. This feature allows the brain to identify potentially important sounds even during sleep.
Thus, nighttime alertness does not mean that a person is constantly in a state of full readiness for danger. Rather, the brain selectively retains sensitivity to signals that may be significant. Therefore, a sudden creaking or knocking can wake us up, even if we would not pay attention to it during the day.
The brain doesn't like uncertainty.
When we hear an incomprehensible sound, the brain does not just register it, but tries to identify the source. He compares the received signal with past experiences, expectations and the situation. In cognitive science, this process is described, in particular, through the concept of predictive information processing: the brain constantly makes assumptions about what is happening and compares them with sensory signals.
The authors of another study published in the journal Nature Reviews Neuroscience explained that perception is formed not only on the basis of information coming from the senses, but also taking into account previous experiences and expectations. Due to this, the brain can interpret incomplete sensory signals and recognize objects and events even with a lack of information. However, previous expectations can also influence the perception of ambiguous signals, changing their interpretation.
For example, there is a knock in the dark. It could have been caused by the wind, the movement of pipes, a neighbor, or a person outside the door — but the sound itself still does not allow us to accurately determine the cause. The brain tries to fill in the missing details and construct the most likely explanation. If a person is already anxious or in an unfamiliar environment, the assumption of danger may have a stronger effect on their perception.
However, the brain does not necessarily consciously choose the worst-case scenario. He is trying to assess what is happening based on incomplete information. In conditions of uncertainty, a defensive reaction can work even when there is no real threat: it is potentially riskier to miss a danger than to play it safe several times.
Why do we immediately think of the worst
An ambiguous signal by itself does not necessarily cause great fear. A lot depends on a person's condition and how safe the situation seems to them. If we are tired, anxious, or already expecting trouble, our attention may shift more to potential threats. In this case, a neutral sound becomes easier to interpret as disturbing.
The price of a possible mistake is also important here. If you hear a rustle in the dark, you can assume that it's the wind, or you can assume that someone has entered the house. In the first case, a person runs the risk of not noticing the real danger, in the second — to be afraid of the usual household sound. From the point of view of a defensive reaction, it can be more dangerous to miss a threat than to play it safe several times. Therefore, the threat detection system is sensitive not only to obvious signs of danger, but also to signals that only potentially indicate it.
Some people may experience a vicious circle of anxiety: fright makes them listen, increased attention highlights new sounds, and they, in turn, increase anxiety. However, such a reaction does not always occur, and an alarming explanation of what you have heard does not necessarily turn out to be correct. The ability to check the source of the noise, turn on the lights and make sure that nothing is happening around helps to reduce uncertainty and interrupt this chain.
When the brain creates sounds by itself
Sometimes night sounds do not arise from an external source. On the border of sleep and wakefulness, a person can hear footsteps, whispers, voices, or other distinct noises. Such experiences are called hypnagogic if they occur upon falling asleep, and hypnopompic if they appear upon awakening. They may be accompanied by visual images and a feeling of movement, such as falling.
One of the most frightening options is exploding head syndrome. A person suddenly hears a very loud sound, similar to an explosion, a shot or a door slamming, although in reality nothing like this happened. Despite the name, it is usually a painless and benign sleep disorder. Its exact causes have not yet been determined.
In a study of the clinical features of exploding head syndrome, published in the journal Sleep Medicine, scientists analyzed the responses of 6,686 people, of whom 3,286 reported having experienced similar episodes at least once. About 44.4% of participants with this condition experienced intense fear during seizures. At the same time, episodes caused clinically significant psychological discomfort in 25%, and interfered with daily life in 10.1%.
Such phenomena are related to the fact that the transition from wakefulness to sleep occurs gradually. Different processes—awareness of the world around us, sensory processing, and dream—related activity-do not always change synchronously. As a result, an internally generated image or sound can be perceived as a real event. Source monitoring also plays an important role: the brain's ability to distinguish between external signals and its own thoughts, memories, or imagined speech. In certain states, this boundary becomes more difficult to recognize.
Where does the feeling of someone else's presence come from
Another unusual experience that can occur at night is the feeling that someone else is in the room. Sometimes it is accompanied by sleep paralysis: a person already realizes that he has woken up, but cannot move for a while. This is because during the REM sleep phase, the brain temporarily suppresses the activity of most skeletal muscles, preventing a person from physically reproducing movements from dreams. In sleep paralysis, consciousness returns before this muscular immobility disappears.
At the same time, dream elements can continue to influence perception. It seems to a person that someone is standing next to him, watching him or approaching. The very feeling of someone else's presence is also being studied outside of sleep paralysis. A study on this phenomenon has been published in The Lancet Psychiatry journal. The authors examined its manifestations in various neurological and mental conditions, anxiety and sleep paralysis, as well as possible cognitive and neurophysiological mechanisms.
One of the experiments helping to understand how such a sensation occurs is described in a study in the journal Current Biology. The participants controlled the robot in front of them with finger movements, and another device behind them repeated these movements and touched their backs. When the researchers introduced a delay between human movement and the robot's touch, some participants had the feeling that someone else was nearby. In some cases, it seemed to them that an invisible person was standing behind them.
The experiment does not explain all cases of sleep paralysis, but it shows how the sensation of another person's presence may depend on how the brain combines signals about movement and touch. Usually, these signals are consistent with each other and help us understand where our own body ends and the world around us begins. If the connection between them is disrupted, the brain may misinterpret the sensations, giving the impression of someone nearby.
Thus, nighttime anxiety does not necessarily mean that a person has heard or felt something real. The brain is constantly building a picture of the world around it based on incomplete signals, expectations, and past experiences. In the dark and on the edge of sleep, this system can malfunction: an ordinary sound seems dangerous, an inner feeling is the presence of an outsider, and a fragment of a dream is an uninvited guest in the room.
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