The Line Between Conscious and Unconscious Just Got Fuzzier

During deep sleep, people can hear, process and respond to verbal input, new experiments reveal

While we sleep, we dream wild, implausible stuff while the mind processes emotions, turns thoughts into memories. During the deepest stage of sleep, the mind shuts down so thoroughly that a physical cleanout of brain toxins takes place, while chemicals are released to restore and rejuvenate the entire body.

We are totally out of it.

Or are we?

Much about sleep remains largely mysterious. Why do we dream? How can people possibly walk, cook, drive or even have sex while sleeping? Heck, people can even learn new words during deep sleep. Clearly the line between consciousness and unconsciousness is fuzzy. And it just got fuzzier.

New research reveals that seemingly anyone can hear and understand verbal instructions while they are asleep and dreaming, and some can do so even in the deepest stage of sleep when brain waves have slowed to a crawl. They are able to listen, process and respond with a smile or a frown, scientists report in the journal Nature Neuroscience.

“Wakefulness and sleep are not stable states,” said study team member Lionel Naccache, a neurologist at the Paris Brain Institute. “On the contrary, we can describe them as a mosaic of conscious and seemingly unconscious moments.”

To understand the novelty and importance of the findings, we need a little background on the research that led to these new experiments.

Hacking into dreams

In previous work conducted by some of Naccache’s colleagues and separate teams in the US, Germany and The Netherlands, scientists experimented on lucid dreamers — people who are aware they are sleeping while dreaming vividly, and can sometimes control the narrative of their own dreams. Lucid dreaming happens during the REM (rapid eye movement) stage of sleep, when dreams are most common.

The scientists hacked into the dreams of these lucid dreamers and got about 20% of them to accurately do some simple math and answer yes-no questions posed verbally by the scientists, responding through eye movements or by contracting facial muscles. Again, all while sleeping. After they woke up, some of the lucid dreamers remembered the questions as being part of their dream.

“We found that individuals in REM sleep can interact with an experimenter and engage in real-time communication,” said the senior author of that study, Ken Paller, PhD, director of the Cognitive Neuroscience Program at Northwestern University. “We also showed that dreamers are capable of comprehending questions, engaging in working-memory operations, and producing answers.”

By some accounts, about half of people say they’ve experienced lucid dreaming, but it seems to be common among only around 1% of the population. For scientists, it has been an open question whether this two-way communication, called dream dialoguing, might be possible with sleeping subjects who were not dreaming lucidly, or not dreaming at all.

Beyond lucid dreaming

The new study aimed to determine if such two-way communication could happen in other stages of sleep, and with people who do not experience lucid dreams.

In a lab, the scientists had 49 volunteers take naps. About half of the participants were deemed normal sleepers — people who aren’t lucid dreamers and who otherwise sleep soundly. The rest of the volunteers had narcolepsy, in which a person may fall asleep suddenly, uncontrollably, during the day. Narcoleptics are also prone to lucid dreaming.

The participants were told to smile if a word they heard was real, and frown if it was made up. Then while they napped — their sleep confirmed by polysomnography, a mix of brain and heart activity, eye movements, and other measures — the scientists yammered on, with real or fake words.

“Most of the participants, whether narcoleptic or not, responded correctly to verbal stimuli while remaining asleep,” study team member Isabelle Arnulf, also of the Paris Brain Institute, said in a statement. “These events were certainly more frequent during lucid dreaming episodes, characterized by a high level of awareness. Still, we observed them occasionally in both groups during all phases of sleep.”

Even in the deepest phase of sleep, when brain activity is minimal and the brain and body are essentially shut down for repair work, the people known for lucid dreaming could hear, process and respond to external stimuli. Good sleepers—the non-narcoleptics—were able to react during all but the deep period of slow-wave sleep.

What’s it all mean?

Talking to people while they are asleep could help researchers learn more about everything from the nature of consciousness to what dreams mean to how our minds make memories. Rather than looking at sleep from the perspective only of how deep it gets, scientists can investigate how alert we are during the night.

“We’ve long known that cognition and consciousness are not shut off during sleep, but our results now broaden the opportunities for empirically peering inside the sleeping mind,” the scientists in the earlier experiments concluded. “Experiments from many corners of cognitive neuroscience can be modified and applied to interactive dreaming, perhaps opening up new ways to address fundamental questions about consciousness.”

Already, other experiments on lucid dreamers have suggested that dreams play out in the mind more like actual perception than pure imagination — something that scientists and philosophers have been debating since ancient Greek times.

The strangeness of dreams and the inability for most of us to control them can resemble schizophrenia and other psychoses, scientists say, so lucid dreaming and dream dialoguing are seen as a potential gateway into better understanding mental illness.

Finally, further investigation into the mysterious phases of existence between sleep and wakefulness could offer insights into disorders that keep people up at night and leave them exhausted during the day, “such as sleepwalking, sleep paralysis, hallucinations, the feeling of not sleeping all night, or on the contrary of being asleep with your eyes open,” Arnulf said.

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