Sleep is known to contribute to the healthy functioning of the brain and the consolidation of memories. Past psychology research specifically highlighted its role in retaining episodic memories, which are memories of specific events or experiences.

Researchers at Rotman Research Institute at Baycrest Academy for Research and Education, University of Toronto and other institutes recently carried out a study to better understand the extent to which sleep transforms how we remember real-world experiences over time and what processes could underpin this transformation. Their findings, published in Nature Human Behaviour, suggest that sleep actively and selectively improves the accuracy with which we remember one-time real-world experiences.

“My lab studies real-life memory such as the memory of events that occur as part of daily experiences,” Brian Levine, senior author of the paper, told Medical Xpress. “We are interested in how these memories are transformed over time and why some elements are remembered while others are forgotten. This is hard to do with naturalistic events in peoples’ lives where we have no control over what happened. So we set up the Baycrest Tour as a controlled but naturalistic event that we could use to test memory.”

Levine and his colleagues have already carried out various studies utilizing the Baycrest Tour as an experimental paradigm, which focused on the accuracy of memories, aging-related changes in memory, and other specific dimensions of memory. The Baycrest Tour is an immersive audio-guided tour of various artworks, which participants observe while walking.

In their recent study, the researchers, including Drs. Nicholas B. Diamond and Stephanie Simpson (then graduate students in Levine’s laboratory), used this immersive art tour to specifically explore how memories of real-world events are transformed during sleep. Notably, this topic was extensively studied using laboratory tasks but has so far not been investigated with naturalistic (i.e., real-world) experiences, such as the Baycrest tour.

“Participants took the Baycrest Tour, which resembles an audio-guided museum tour, followed by tests assessing memory for details (e.g., the color of a painting) or sequences (e.g., whether a particular painting came before or after another item on the tour),” said Levine. “They were tested right after the tour and again the next day, a week later, and a month later. We tested memory for items vs. sequences at each time interval.”

Interestingly, people remembered the sequence of the tour better after a single night of sleep, with the advantage of sequence over detail memory persisting in subsequent tests.

“This is a big deal because memory generally declines over time, which is what happened for detail memory,” said Levine. “When sleep preserves memory, it is usually protecting it against decline. So, our study shows that, under some conditions, the effect of sleep actively enhances memory, rather than just protecting it from decline.”

The researchers repeated their experiment with 77 participants. 39 of these participants took part in the tour at night, then completed a memory test, slept in a sleep lab, where their brain activity was monitored, and then completed the same memory test again after sleeping. The remaining 38 took part in the tour in the morning, which was immediately followed by the same memory test, to complete the test again in the evening after being awake for the whole day.

“Both groups then went on to do subsequent tests at one week, one month and one year. As in the first study, sleep boosted memory for sequences and not details, and the difference lasted over one year. For the participants who stayed awake after both sequence and detail memory declined to a similar degree, which means even one night of sleep can benefit sequence memory up to a year later,” said Levine.

Levine and his colleagues ultimately analyzed the memory test results in conjunction with the brain activity of the participants in the sleep conditions. Overall, the findings of this recent study suggest that sleep not only preserves people’s memories of real-life experiences for long periods, but it can also enhance the accuracy with which they are remembered.

This interesting observation could inspire other researchers to probe the effects of sleep on memory of specific sequences of events. Meanwhile, Levine and his colleagues plan to continue investigating the effect they uncovered using other memory tests and experimental paradigms.

“We’d like to know if this effect occurs for other ways of testing memory, such as free recall, as opposed to recognition memory that we tested for in this study,” added Levine. “We’re also interested in the effects of sleep on emotional or traumatic memory.”

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