When dreams draw on our memories

If we have known for a long time that restorative sleep is beneficial for memory, the role of dreams is still quite mysterious for scientists. A team of Quebec and American researchers has tried to provide the beginning of an answer to this question by decoding the content of our dreams to link them to more or less distant memories.

When we remember them when we wake up, some dreams can seem very strange to us, even completely invented. But many could rely on very real memories. In any case, this is one of the conclusions of a study published in the journal sleeping last April.

“More than 85% of the dreams we collected during this study were associated with a memory or an element of the dreamer’s waking life, such as a concern, for example,” explains Claudia Picard-Deland, lead author of the article.

“There could even be several sources of memory within the same dream. The majority had at least two, and it could go up to ten”, specifies the postdoctoral researcher at the Center for Advanced Studies in Sleep Medicine at the University of Montreal.

A memorable night

This collection of dreams was carried out by the researcher and her team during an experiment involving around twenty participants, who came to spend the night at the Medical Center of the University of Rochester, in the United States. “We woke them up about 12 times during the night to collect their dreams,” she explains.

The participants each time recounted their dreams – if they remembered them – before going back to sleep until the next awakening. “So it wasn’t a very pleasant night for them, but they were willing and motivated,” she says with a smile.

Claudia Picard-Deland wanted to collect dreams at the beginning, middle and end of the night, as well as in all stages of sleep.

When we sleep, we go through several sleep cycles that last about 1 hour 30 minutes. Each cycle is made up of several successive stages: falling asleep, light slow-wave sleep, deep slow-wave sleep and paradoxical sleep. And if the latter is traditionally referred to as the realm of dreams, we can also dream during the other stages of sleep.

At the beginning of a cycle, during the sleep phase, one can have very fleeting dreams. “It can be fast or abstract images, sound or visual hallucinations, colors, movements, strange bodily sensations”, illustrates the researcher. Conversely, “dreams in REM sleep are much longer, richer and more complex, but also more emotional”.

“In light slow wave sleep, dreams are a little less immersive, with less sensation, and less emotional as well,” she adds. And in deep slow sleep, the dreams are short, a little fuzzy. Sometimes we dream of more abstract things, or more conceptual ones. »

More or less distant memories

In the early morning, participants were asked to connect the eight dreams they had on average during the night to situations, emotions or events they may have experienced before.

More than half of the participants’ dreams drew on their memories of the previous week. And the more we progressed in the night, the more old memories resurfaced.

“You would think that at the beginning of the night, we deal with a recent event in a more direct way. Then, the more we progress towards the end of the night, the more we associate it with distant memories, to quietly integrate it into our memory,” she explains.

It would be a way for the brain to store a recent event in the right place in its vast library of past memories.

But when several memories are intertwined, the analysis can be more complex for scientists. “The further we move away from a specific memory, the more difficult it can become to make the distinction with a new creation”, underlines Perrine Ruby, researcher at the Neuroscience Research Center in Lyon, France.

Emotions and nightmares

And what about the slightly strange dreams, which can hardly be linked to a memory?

“Some say they prepare us to face unfamiliar situations,” replies Karim Benchenane, researcher at ESPCI Paris, France. “Being attacked by zebras has never happened to me. But I dreamed about it last night, so if I ever face it, I’ll be less surprised,” he jokes.

Other research groups, such as that of Perrine Ruby, believe that these dreams help us regulate our emotions.

“But it can also show you a new perspective: you can realize that, faced with this tsunami that happened to you, you weren’t so scared. It can question you on the bottom of things”, nuance M^me Ruby.

This emotional regulation also has its limits. “If there is too much emotional intensity, regulation can fail and it can lead to a nightmare,” adds the researcher, who is also the author of a review of the literature on this subject.

Claudia Picard-Deland is convinced that this research on the link between dreams and memories could help us better understand how our brain works.

“Sleep does not simply strengthen memory,” says the researcher. In everyday life, you don’t want to remember everything, like what you had for lunch a month ago. We want to extract from memory what is most important, to make new associations between things. We want to be able to use our past experiences to better anticipate the future and better act in the world when we wake up. Sleep could play a role in this, and dreams could help us better understand these mechanisms. »