Researchers at the Netherlands Institute for Neuroscience have uncovered new insights into what happens inside the brain of someone who is “stuck” between sleep and wakefulness. Their study, published in Nature Communications, found that people who report conscious experiences during parasomnia episodes—such as sleepwalking—show specific patterns of brain activity. These patterns are similar to those seen during dreaming, suggesting that the physiological processes of parasomnia and dreaming might be closely linked.
Parasomnias, including sleepwalking and sleep terrors, are mysterious conditions that occur during non-rapid eye movement (NREM) sleep. These conditions can lead to complex behaviors while the person remains partially asleep. Individuals may sit up, talk, leave their bed, or even perform dangerous actions like driving or committing acts of violence, raising significant questions about consciousness and responsibility.
“Affected individuals can hurt themselves or others during episodes and may later feel deeply embarrassed for what they did,” said senior author Francesca Siclari, head of the dreams lab at the Netherlands Institute for Neuroscience.
Understanding what people experience during these episodes is crucial for both medical and legal reasons, but until now, much of what we know has been based on anecdotal reports and retrospective accounts.
For their study, the researchers recruited twenty-two patients who had disorders of arousal. These participants underwent detailed medical evaluations and two high-density electroencephalography (hd-EEG) sleep recordings. The first recording was done during a regular night of sleep, and the second after a night of total sleep deprivation designed to increase the likelihood and complexity of parasomnia episodes.
During the second recording, researchers played loud sounds to provoke parasomnia episodes. When an episode occurred, the experimenter waited for it to end before calling the patient’s name and conducting a semi-structured interview about their experiences. This immediate questioning aimed to capture the patient’s most recent experiences before they could forget them.
Three experts independently reviewed video recordings of these episodes to classify them as either parasomnia or normal awakenings. Only episodes unanimously agreed upon as parasomnia by all three experts were included in the analysis. The researchers then compared the brain activity patterns during episodes where patients reported conscious experiences to those where no experiences were reported.
The study recorded 102 potential parasomnia episodes, of which 75 were unanimously confirmed by independent experts. These episodes primarily consisted of short confusional arousals, with none of the participants leaving their beds. The episodes varied in length, with an average duration of approximately 28 seconds.
Detailed analysis showed that conscious experiences included a wide range of scenarios, from ordinary thoughts to complex, constructed scenarios often involving perceived threats or delusions. In some cases, these experiences were coherent with the behaviors observed during the episodes, such as interacting with imaginary objects or responding to perceived dangers.
“It was often about an impending misfortune or danger,” Siclari explained. “Some reported that they thought the ceiling was going to come down. One patient thought they’d lost their baby and was searching through the bedsheets, and stood up in bed to try to save ladybugs from gliding down the wall and dying. In 19% of the cases, the patients weren’t experiencing anything and simply awoke to find themselves doing things, almost like a trance.”
The researchers found that reports of conscious experiences were significantly higher when high-amplitude EEG slow waves were present in the anterior cortical regions, coupled with activation in the posterior cortical regions. These patterns are similar to those observed during dreaming. Additionally, the ability to recall the content of these experiences was linked to higher EEG activation in the right medial temporal region before movement onset.
“Compared to patients who did not experience anything, patients who dreamt during the episode showed activations that were similar to brain activations previously found for dreaming, both immediately before the episode, and also during the episode,” Siclari said.
“What determines whether the patient will be completely unconscious or will instead dream seems to depend on the state the patient is in at that moment. If we activate the brain while they’re likely already dreaming, they appear to be able to ‘make something’ of the activation, while when their brain is largely ‘inactivated’, simple behaviors seem to occur without experience.”
“Interestingly, patients almost never mention the sound that initiated the parasomnia episode, but rather some other type of impending danger. The louder we go with the sound volume, the higher the chance that we provoke an episode.”
Despite these intriguing findings, the study has some limitations to consider. The relatively small sample size limits the generalizability of the results, and the focus on confusional arousals means that other types of parasomnias were not represented. Additionally, the analysis was based on fixed-length episodes, which might not capture the full complexity of brain activity associated with varying episode durations.
Future research could address these limitations by involving larger and more diverse participant groups, including those who experience parasomnias during rapid eye movement (REM) sleep. Studies conducted in natural home environments could also provide more comprehensive data on parasomnia episodes. By exploring these avenues, researchers hope to gain a deeper understanding of the neural systems involved in different types of parasomnias.
Siclari emphasized the potential impact of this work, both in advancing scientific knowledge and in practical applications. “These experiences are very real to the patients and most already felt relieved to be sharing them with us,” she said. “our research clarifies what they are experiencing, which is educationally valuable. Additionally, our work could contribute to more specific drug interventions in the future.”
“Parasomnias are often treated with unspecific sleeping drugs, which isn’t always effective and can have negative side effects. If we can deduce which neural system is working abnormally, we can eventually try to develop more specific treatments.”
The study, “Shared EEG correlates between non-REM parasomnia experiences and dreams,” was authored by Jacinthe Cataldi, Aurélie M. Stephan, José Haba-Rubio, and Francesca Siclari.