Sleep enhances memory for life events over the long term

While sleep's positive impact on cognition has long been recognized, Baycrest researchers have revealed that a good night's sleep may actively enhance memory rather than simply protect it from decline. The new findings are the first to reveal that sleep helps us better remember the order of events in our lives, even after more than a year.

"While our memory for features such as object size and colour declines over time, sleep can improve our memory for event sequence," says Dr. Brian Levine, Senior Scientist at the Rotman Research Institute, part of the Baycrest Academy for Research and Education (BARE) and senior author on this study, titled "Sleep selectively and durably enhances memory for the sequence of real-world experiences" and published recently in Nature Human Behaviour. "This study deepens our understanding of how critical sleep is for integrating experiences into memory."

Most studies on human memory for events require participants to memorize items, such as words or pictures, in a laboratory setting. However, in this study, Dr. Levine-along with then-graduate students Dr. Nicholas B. Diamond and Dr. Stephanie Simpson-designed a unique and immersive real-world experience in a 20-minute audio-guided tour of artworks displayed at Baycrest, a global leader in aging and brain health in Toronto, Ontario. Participants took the tour and were tested on their memory at five delay intervals ranging from one hour to 15 months after the tour. The tests included questions on the physical features of the artwork and the sequential order of tour items. The researchers found that sleep boosted memory for sequences, whereas memory for features declined.

To confirm their findings, they ran a second study where participants were randomized to either a wake group or a sleep group. The sleep group did the tour and first memory test in the evening and the second memory test after a night of sleep in a sleep laboratory with brain activity monitored with electroencephalography (EEG). The wake group did the tour and first memory test in the morning and the second memory test in the evening after normal daily activities. To test the long-term effects of sleep on memory, both groups completed additional memory tests one week, one month and 15 months following the tour.

Main study findings:

• Sleep improved memory for the order of the tour items (sequences), but not memory for features.
• The advantage of sleep versus wake for sequential memory was evident after a single night of sleep, and this advantage held at all test points, including 15 months later.
• No such improvement was found for memory for features of the tour items, such as shape and colour, which continued to decline over time.
• Brain activity recordings in the sleep laboratory showed that specific brain waves during deep sleep – namely slow waves and spindles – were linked to overall memory enhancement.

Good sleep is an essential part of a healthy lifestyle, including for the brain and cognition. This study shows how deep sleep benefits memory for the sequential structure of everyday events, even when the specific details of those events fade with time, and points to a potential cause of memory changes in older adults living with dementia. "The benefits of sleep on memory are powerful; just one night makes a difference that persists over a year," said Dr. Levine.

This study was funded by the Canadian Institutes of Health Research.