In the no-man’s-land between going to sleep and waking up, your brain goes through myriad states. On an electroencephalogram (EEG), or a readout of the brain’s electrical activity, scientists and doctors can tell each stage apart. But it is not totally clear what each one brings to the mix. Sleep relieves fatigue, allows the brain to process the events of the day, and appears to aid in memory and learning. But which stage does what? Answering that question may reveal how to get better quality sleep.
To help gain insight, scientists conduct serial awakenings, a type of lab experiment where they repeatedly wake up sleeping people through the course of a night and ask them to fill out questionnaires. (If that sounds like your personal nightmare, thank all the people who are doing it for science.) A new study published in the journal PLOS Biology focuses on awakenings during a stage called non-rapid-eye-movement sleep 2, or NREM2 for short, which tends to make up about half of all our time asleep. Researchers found something intriguing: When people were woken from immersive dreams, which on EEG look a lot like being awake, they nevertheless reported having felt deeply asleep. This suggests, the researchers speculate, that vivid dreams in this sleep phase may be contributing to a feeling of having slept deeply.
What’s the purpose of dreaming?
Dreaming is such a common experience that it may seem surprising that scientists do not yet understand why we do it. One approach to studying dreams is to try to alter them by playing sleepers certain sounds or by waking them up and letting them sleep again, and looking at whether these changes affect how they feel later.
“We try to modulate brain activity, and we try to have an effect on how subjects feel subjectively, in terms of sleep depth and what they experience during sleep,” says Dr. Giulio Bernardi, a professor at IMT School for Advanced Studies Lucca in Italy and an author of the new paper.
In this new study, he and his colleagues investigated NREM2 sleep, a phase that can include a variety of states, including immersive dreams, simple dreams, and no dreams at all. The team awakened 44 sleepers in NREM2 and had them report what was going through their minds, including whether they had been dreaming. Sometimes they had been, but had no memory of what it contained—a so-called “white dream.” If they remembered the dream, they were asked to rate its vividness and immersiveness, among other features. They were also asked to rate how deeply they had been asleep.
The deepest subjective sleep occurred mainly when the EEG showed people had been deeply unconscious and the electrical patterns that represent consciousness disappeared.
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But people also felt they’d been deeply asleep when they reported immersive dreams, although their EEG patterns looked more conscious.
“The brain can become more active, more awake, and still we can feel that we are deeply asleep if we are dreaming,” says Bernardi.
What that means is still a puzzle
Could dreaming make the difference between a restless night of sleep and a satisfying one? This study isn’t set up to tell us that, since it doesn’t address restfulness in the morning. Given how often people were awakened, it might be hard to tease apart the influence of dreaming and the influence of multiple awakenings on how sleepers felt.
But the study does suggest the question of how dreams are linked to sleep depth is worthy of further research.
Karen Konkoly, who studies dreaming at the University of Cambridge in the U.K. and was not involved in this work, has herself participated in a similar study. “It was fascinating to observe my mind at different times of night and realize how often I actually didn’t feel like I’d been fully asleep,” she wrote in an email.
“So to me, the best way to tell that I was in fact deeply asleep is when I was awoken from an immersive dream. I knew I was asleep because I was dreaming. That is my best intuition explaining this finding, that it’s difficult to tell how deeply asleep one is, and an immersive dream is one clear way to answer that question.”
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Bernardi thinks there may be more to it. “While dreaming does indicate sleep, it does not necessarily imply ‘deep’ sleep per se,” he wrote in an email. “What I find particularly telling in our work is that participants sometimes reported having been deeply asleep even when they could not recall any specific dream content; only a general impression of having been in a rich or immersive state.”
Maybe, he speculates, that feeling of depth comes from a sense of disconnection with the outside world. That’s something you could get from being completely unconscious or perhaps from a dream so immersive that the external reality doesn’t penetrate.
The study is a reminder that there is more to sleep than simply being unconscious. Perceptions can be powerful: For instance, many people who report insomnia are in fact asleep much of the night. What seems to have gone wrong in those cases is that their perception of having been asleep is knocked askew.
Sometimes dreams can feel a little too real, though. People who experience a phenomenon known as “epic dreaming” report having dreamt all night, often performing repetitive tasks in mind-numbing detail.
“In the end, when they wake up in the morning, they are super-tired, as if they worked for the whole night,” says Bernardi. “So dreams probably have to stay in the right range, let’s say, of immersivity—otherwise they become negative for our sleep.”
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