Sleep is generally perceived as a period during which the body and mind are at rest, as if disconnected from the world. However, recent research shows that people can take in information, respond with smiles and gestures to verbal stimuli, and interact during almost all stages of sleep. Researchers have described these exchange cycles as windows to the outside. The study has been published in the journal Nature.
Although it may seem familiar because it takes place every night, sleep is a very complex phenomenon. Its definition indicates that awareness of the environment stops and it becomes impossible to react to the outside world, but the border between wakefulness and sleep is more porous than it seems. To demonstrate this, the study authors asked 49 people, 27 with narcolepsy — who fall asleep suddenly — and 22 without a sleep disorder, to take a nap. While they slept they were monitored with polysomnography equipment that tracks brain and heart activity, along with muscle and eye movement. They were asked to react with a smile or a frown if they were able to hear during sleep. Some of the words that were spoken to them made sense and others didn’t. Both the narcoleptic group and the other subjects followed the instruction. In some cases they smiled on hearing the invented words and made gestures of confusion with the real ones, and vice versa.
Delphine Oudiette, a cognitive neuroscience researcher and co-author of the study, admits she was surprised by the results: “You’re not supposed to be able to do that while you sleep.” This ability occurs intermittently, with a pattern of brain activity that predicted whether a stimulus would be followed by a response or not, “as if a window was sometimes open to perceive the outside world, and at other times closed.” Neuroscientist Lionel Naccache, who also participated in the study, told Nature that the research shows that wakefulness and sleep are not stable states: “On the contrary, they can be described as a mosaic of moments, both conscious and apparently unconscious.”
Oudiette began exploring these windows after an experiment she and her team conducted in which they were able to communicate with people who are aware that they are dreaming while they sleep, also known as lucid dreamers. They asked the subjects questions and received answers through eye and facial muscle movements. In their most recent research, the team included people with narcolepsy, as they tend to be lucid dreamers. They expected to interact with them, but what was surprising was the response of people who do not suffer from this disorder, or who are not habitual lucid dreamers.
Neuropsychologist and expert in sleep medicine Francisco Segarra describes the research as “a very well-designed and controlled study” due to the way the subjects’ reactions were measured. He highlights that it was not clear whether it is possible to process information during non-shallow sleep. Both in phase one, which is the dream state known as wakefulness, and in phase two, the people studied reacted with smiles and gestures.
The findings suggest that it is possible to develop communication protocols to better understand how mental activity changes during sleep. In this way, scientists could begin to identify the parts of the brain that are active during rest and how they relate to consciousness. Oudiette explains that a possible application of the discovery is to access the cognitive processes of sleep, both normal and pathological, such as cases of insomnia and sleepwalking.
People who reported being unable to sleep recorded normal sleep during the study. Oudiette considers that the discrepancy between the subjective and objective sleep of people who suffer from insomnia could be explained “by a greater connection with the outside world.” She has described sleepwalking as an exacerbated version of those windows. Her hypothesis is coherent, she points out, due to the fact that sounds often trigger episodes of sleepwalking.
Sleep experts have tried to discover whether it is possible to reinforce some types of learning during rest. Although it is not the objective of Oudiette’s study, she believes that it is possible to test whether sending useful information to sleepers during brief periods of connection with the outside world helps them remember it better. Other studies have shown that new associations can be learned in phase one of sleep and can even influence behavior, for example, by reducing smoking through sensory stimuli that are processed unconsciously during rest.
Segarra agrees that the discovery can provide clues and represents “one more step” in expanding the understanding of dreams, as it remains a subject about which “very little is known.”
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