Psychologists Find Sleep Can Distort Our Memories

While having a good night's sleep might help you to remember things you're trying to remember, it can also help our brains make up entirely false memories.

The human brain's memory is notoriously unreliable, often missing things that were glaringly obvious or remembering things happening that never actually did. New research in the journal Royal Society Open Science reveals that sleep might help us remember things, and also remember false memories.

These false memories often arise when people are given a list of related words to memorize, and falsely remember a word being there that would have fit the category but in fact was missing.

"We found that participants had better memory for the lists in terms of better recall of the words in the lists. But their errors were also revealing—they made fewer random errors (intrusions), and more errors that suggest that they had learned the gist of the lists," Gareth Gaskell, a professor of sleep psychology at the University of York in England, told Newsweek.

The researchers tested 488 participants on their ability to recall a list of words 12 hours after seeing them, with some of the participants being allowed to sleep in the 12-hour interim.

They found that those who had slept remembered more of the words on the list than those who had not, but they were also more likely to give words that weren't on the list, but were related. The related incorrect words are known as "lure words," while completely unrelated incorrect words are known as "intrusions."

"The wordlists presented were designed to induce so-called 'semantic false memory.' For instance, after seeing "nurse," "hospital," "sick," and "medicine," participants may say that 'doctor' was presented, even though it never was—the word "doctor" is referred to as the critical lures throughout our paper," Matthew Mak, study co-author and researcher at the University of York, told Newsweek.

"Such false memory arises because, 1) 'Doctor' presumably represents the gist of the wordlist, and humans are known to often rely on gist encoding (i.e., remembering the core meaning, but not necessarily the precise details). 2) Words in our mind are stored in association with each other, so seeing those medical-related words may automatically activate 'doctor', making participants felt like they had seen this word before."

"In our study, participants first remembered lists of related words—e.g., nurse, hospital, sick, medicine. Twelve hours later, participants completed a free recall task, such as writing down all the words you can remember. We found that participants who had a sleep opportunity in those 12 hours vs. those who stayed awake recalled more of the presented words, in line with decades of evidence that sleep is beneficial to memory retention," Mak said.

This suggests that sleep has a complex role in memory, influencing not only how well memories are retained but also potentially the nature of the memory.

"Memories in some ways are more about our future than our past. What we want is knowledge about our past that can be applied in a generalized way to help us to deal with future events," Gaskell said.

"Future events won't be identical to the past events, so a gist-like representation might actually be more useful than a 'perfect' detailed representation. So what sleep might be doing is helping us to store memories in a gist-like way that can then be better applied to our future interactions."

The researchers also found that the results varied based on the time of day that the participants were remembering the list, with both groups suggesting more incorrect and unrelated words in the evening.

"We found an unexpected time-of-day effect, such that completing free recall in the evening led to more intrusions—neither studied nor lure words," the authors describe in the paper.

"Above and beyond this time-of-day effect, the sleep participants produced fewer intrusions than their wake counterparts. When this was statistically controlled for, the sleep participants falsely produced more critical lures. They also correctly recalled more studied words, regardless of intrusions."

The authors do recognize several limitations of their study, namely that all participants were aged between 18 and 25, and that the tests were performed online, meaning that other distractions and environments could not be controlled.

However, they hope that their research paves the way to new discoveries regarding sleep's role in memory.

"Our study provides a rich new body of evidence to help determine the contribution of sleep," they wrote.

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Update 12/5/2023 12:32 ET: This article was updated to include comment from Mak.