A new study published in Nature Human Behaviour suggests that sleep doesn’t just protect memories from fading—it transforms them. Researchers found that sleep selectively strengthens memory for the sequence of real-world events while allowing many of the perceptual details to fade. This transformation wasn’t short-lived: even 15 months after a one-time event, people better remembered the order in which things occurred—if they had slept soon after the experience.
The study addresses a long-standing question in memory research: does sleep simply preserve memories by protecting them from interference, or does it actively enhance and reshape them? Previous studies have shown that sleeping after learning improves memory, but many of those studies used artificial tasks like memorizing word lists or images on a screen. Researchers working in Brian Levine’s research lab at Baycrest’s Rotman Research Institute took a different approach by examining memory for a real-world experience and tracking how it changed over time—days, weeks, and even more than a year later.
“For me, I’ve had a long-standing interest in how we remember complex personal past events (episodic memory),” said co-first author Stephanie Simpson, who is now a postdoctoral fellow at the Centre for Addiction and Mental Health. “When I joined the lab as a master’s student, I started reading about how factors like sleep can shape the way humans retrieve information, but to my knowledge most of these studies tested how people remembered words or simple images on a computer screen. Furthermore, most of these studies didn’t explore whether the effects of sleep on memory were long-lasting.”
“I was motivated to understand how the existing research I read about that was conducted in conventional, highly controlled experimental designs would extend to the ‘noisier’ real world. I was drawn to questions like: How can we bridge the study of fundamental research concepts into more naturalistic settings?”
“Like Steph, I’d always been fascinated by episodic memory – our ability to mentally time travel from the present to specific experiences from our past, reliving some amount of the perceptual, emotional, or contextual elements of that past event. Anything from the experience of eating a mediocre sandwich yesterday to my 10th birthday party,” added co-first author Nicholas Diamond, who currently works for the Government of Canada.
“One reason I’m interested is that this ability is disproportionately affected by some neuropathological diseases like Alzheimer’s. But episodic memory is also just cool and magical on its face, and in my opinion still pretty poorly understood at psychological and neurophysiological levels, even after 150 years or so of empirical research.”
“Some big, basic questions remain: how do single experiences – even non-exceptional ones – stick in our brains for delays beyond a few minutes? When we remember such experiences after a day or a month or a year, are we just making an inference about what happened based on our general knowledge, or can we really re-experience some amount of that original event? And lastly – we know that sleep helps memories stick in our brains, but how exactly does it transform our memories?”
“Saying a memory is ‘better’ or ‘worse’ isn’t really satisfying – I wanted to know how sleep makes a given memory different, and why does this difference lead to that memory being stickier over the long run?” Diamond continued. “I think our answers to these types of questions have been limited by the customs and conveniences of laboratory memory studies – artificial stimuli, delays on the order of seconds or minutes, and tests that assume something is simply remembered or not. We wanted to know how memory for rich real-life experiences – and different aspects of these experiences – changes on the order of weeks, months, and years.”
The researchers created a 20-minute audio-guided art tour through a Toronto healthcare center, where participants listened to descriptions of 33 artworks in a fixed sequence while walking through the space. The tour was designed to mimic how people naturally encounter and encode events in everyday life. After completing the tour, participants were tested on two aspects of memory: the details of each artwork (its color or shape, for example), and the sequence in which they encountered them. The same participants were tested again after sleeping, after a week, after a month, and in some cases, after 15 months.
While overall memory accuracy declined over time, people consistently remembered the order of events better than the specific details if they had gotten a full night’s sleep. In contrast, those who stayed awake during the same post-learning period showed no such advantage. In fact, only the sleep group demonstrated an actual improvement in sequence memory from before to after sleep, a rare finding that runs counter to the typical trend of gradual memory decay.
“Sleep can transform the way we remember complex, personal past experience,” Simpson told PsyPost. “Compared to a period of wakefulness, sleep enhances memory for the order of events from an art tour, but not the details of the event itself.”
To explore how sleep might be doing this, the researchers measured brain activity during sleep in a subgroup of participants using polysomnography, a technique that records electrical activity in the brain. They found that people who experienced more slow-wave sleep—the deep sleep stage associated with memory consolidation—showed stronger memory enhancement.
Even more telling, the degree of improvement in sequence memory was linked to a specific sleep phenomenon known as spindle–slow wave coupling. This occurs when two brain rhythms—sleep spindles and slow oscillations—sync up during deep sleep, creating conditions that support the reactivation and reorganization of memories.
“To help understand why and how sleep transforms memory, after completing our experimental art tour, we recorded people’s brains while they were sleeping using scalp electroencephalography (EEG),” Diamond explained. “Our results showed that one particular part of the sleep cycle – slow-wave sleep – was linked to sleep-related memory improvement. And more specifically, it was the synchronized occurrences of two particular brain rhythms occurring during slow-wave sleep – sleep spindles and slow waves – that was significantly related to overnight memory improvement.”
Interestingly, the sleep-related memory benefit was specific to sequential information. Participants’ memory for featural details—such as what material an artwork was made of—tended to fade over time, regardless of whether they had slept. This selective retention supports the idea that sleep may prioritize certain aspects of memory over others, perhaps because remembering the order of events is more useful for planning and understanding how experiences unfold over time.
“This is some of the first evidence that sleep can actually improve episodic memory – not just reduce forgetting,” Diamond told PsyPost. “In other words: people actually remember a given experience more accurately after a 12 or 24 hour delay (but only if they slept) than they did immediately after the experience. You can think of this as ‘anti-forgetting’ – memory getting better with time. The catch is that it’s one specific element of episodic memory that improves – sequence memory, or memory for the order in which experiences unfold.”
The long-term nature of the findings is especially compelling. Memory for sequences remained significantly better than for details even after 15 months, as long as participants had slept shortly after the event. This suggests that a single night of sleep can have enduring effects on how a memory is stored and retrieved.
“The sleep-related advantage for sequences is durable – it endured over 1 year later,” Simpson said.
The idea that the brain reshapes memories during sleep is not new. In animal studies, researchers have observed that the hippocampus—a brain region critical for memory—replays patterns of activity related to recent experiences, sometimes at a faster pace, during sleep. These replays are thought to strengthen connections between neurons and transfer memories to more permanent storage areas in the cortex. The current study provides evidence that a similar process occurs in humans, and that it may specifically benefit our ability to remember the order in which things happen.
“Why does sleep specifically benefit memory for the sequence of our experiences? We think that this might have something to do with a brain phenomenon called ‘neural replay’, which has been heavily studied in rodents, and only more recently in humans,” Diamond said. “The idea is that, while we’re sleeping, patterns of neural firing that occurred during our waking life repeatedly ‘replay’ in a super time-compressed way, like a movie on fast forward.”
“This may strengthen neural connections between memories for sequences of events, bridging temporal gaps. We didn’t actually measure neural replay – you need to stick wires in people’s brains to do that with high fidelity – but we do know that replay occurs during slow-wave sleep and especially during spindle-slow wave coupling, which is why we expected that to be the key neural marker of memory transformation, which we indeed found to be the case.”
To ensure the findings weren’t just a fluke or the result of test design, the researchers replicated their results across multiple groups. One group slept after the tour, another stayed awake for 12 hours before their second test, and a third had their brain activity monitored during sleep. Across all groups, the same pattern emerged: sleep helped participants retain the sequence of events better than those who stayed awake, and this effect was strongest among those who spent more time in deep sleep and showed more spindle–slow wave coupling.
The study’s design also controlled for several possible confounds. For example, memory tests were matched in difficulty, and test items were randomly assigned and counterbalanced to avoid learning effects. Participants were excluded if they had sleep disorders, neurological conditions, or prior exposure to the tour site. These precautions strengthened the validity of the results.
Still, the study has limitations. While the authors found clear associations between sleep physiology and memory performance, the study cannot definitively establish causality. It’s also possible that individual differences in learning ability or motivation influenced how much benefit people got from sleep.
“A general issue that can not only apply to this study, but the whole field, is relying on a convenience sample of primarily young undergraduate students,” Simpson noted. “These samples tend to overrepresent people who are White, westernized, and well-educated. It would be great to test this in a sample of more diverse adults from a variety of ethnic and socioeconomic backgrounds.”
“In this study we tested how one night of sleep (within the confines of a sleep lab) influenced subsequent memory performance. In an ideal world without financial restrictions, we would investigate how participants slept over multiple nights to get an even more reliable measure of their sleeping brain activity. Alternatively, researchers could opt to use more portable devices (e.g. actigraphy watches) that can be worn at home over multiple nights. However, this comes at the cost of your signal to noise ratio.”
Future studies could explore how these sleep-related memory effects change across the lifespan, especially given that both memory and sleep patterns shift with age. Older adults often experience less slow-wave sleep and more fragmented sleep overall, which may contribute to difficulties with memory for the order of events.
“With age, both episodic memory and sleep habits change considerably,” Simpson explained. “We know that there is a strong connection between sleep and cognitive aging. For example, people with chronic sleep problems are at greater risk of developing Alzheimer’s disease. I think an interesting area of future research would be to explore sleep mechanisms as a biomarker for neurodegenerative diseases marked by significant memory impairment and determine if using sleep alongside memory measures may aid in the earlier diagnosis of individuals at risk for Alzheimer’s disease.”
“Memory for the sequence, or temporal order, of our experiences seems to be a key organizing principle of our real-life memories, sticking around in our brains even for years after a single experience,” Diamond concluded. “And sleep – especially slow-wave sleep – is the key ingredient for boosting sequence memory. So, make sure you get your sleep!”
The study, “Sleep selectively and durably enhances memory for the sequence of real-world experiences,” was authored by N. B. Diamond, S. Simpson, D. Baena, B. Murray, S. Fogel, and B. Levine.
