It’s not quantity but quality that matters when it comes to how much sleep strengthens our memories. A growing body of research is finding that specific stages of sleep shape particular types of learning in the brain. Whether for children napping or for older adults catching z’s at night, we all rely on sleep to consolidate memories from the day.
“As the literature grows, a clearer picture is emerging of which memories are processed during sleep and the relevant aspects of sleep physiology,” says Rebecca Spencer of the University of Massachusetts, Amherst. “Different sleep stages are associated with different forms of memory – meaning, sleep is not singular just as memory is not singular.”
Spencer’s laboratory made headlines last month when they reported in Proceedings of the National Academy of Sciences (PNAS) that daytime naps enhance learning in preschoolers. Children, aged 3 to 5 years old, who napped performed significantly better on a visual-spatial task after the nap and the next day than when they stayed awake during naptime.
Prior to the PNAS study, a few studies had found that overnight sleep was beneficial to memory in children 6 and older. But most of these studies looked at procedural learning – a type of long-term memory used in everything from tying shoes to reading “We knew nothing about declarative learning” – used for route memorization – “and little or nothing about naps in particular other than a study showing a correlation between cognitive performance on standardized measures and time spent napping over a week.”
Importantly, the preschooler study looked not only at cognitive performance pre- and post-naps but also at the physiology of daytime sleep. They found that the density of “sleep spindles” increased during naps, and seemed responsible for the enhanced learning in a visuo-spatial task.
Sleep spindles are high-frequency bursts that appear in the EEG record during non-REM sleep. “They are thought to reflect plasticity, when the brain is really susceptible to making new memories,” Spencer says. And they occur at the same time as replay in the hippocampus, when neural activity associated with an experience while awake repeats during sleep. “So just like if you wanted to learn all the words to your favorite song, you might put the song on repeat and listen to it over-and-over, it seems the brain is doing this as well to learn recently encoded memories,” Spencer explains.
The length of the nap did not matter for memory per se, but the longer a child napped, the more sleep spindles tended to occur. The distributed sleep – nap plus overnight sleep – may be important for younger children, as they have less capacity in the hippocampus than adults to hold information before something interferes with its remembrance.
“But what was interesting to us is that there is nearly no REM sleep in these naps,” Spencer says. Past work has shown that REM is important for emotional memory processing and perhaps also for creativity and other cognitive functions. But as the naps lack REM, Spencer says that they may do little for emotional processing and those other functions.
Although the study only looked at preschoolers, Spencer says that the new results, as well as past work, point to a broader role of spindle density for declarative learning in adults as well. She points to other studies that suggest that sleep spindles are important for procedural learning.
Spencer’s team also has been investigating ways to boost the learning that occurs during sleep. In an upcoming study to be published in Plos ONE, they examined whether sleep prioritized information for learning when the information holds the promise of a future reward. Other studies that looked at just learning (without the sleep component) had found that the brain sorts information based on the amount of potential reward it brings, so she and colleagues had hypothesized that our brains would similarly prioritize information for learning while we sleep.
Surprisingly, they found that sleep equally prioritized all items for learning that had future relevance. “This study shows that sleep further enhances the boost that rewards give to learning and makes the point that the reward doesn’t even have to be a big – any size reward gave that boost,” Spencer says. “The average person appreciates the value of reward for learning: We use this for everything to teach children – sticker charts – and treat addiction.”
Spencer’s advice to young and old about sleep? “First, for young children and parents, my advice is to encourage napping. We don’t yet know what age a child ‘grows out of’ napping and we may mistake naptime sleep defiance as the child no longer needing a nap, yet we don’t think that the child is done needing overnight sleep when they are in a nighttime sleep defiant phase.” Indeed, her team found that many children in their study who only occasionally napped were able to nap when encouraged to do so.
“For older children and adults, my advice is to practice good sleep habits,” Spencer says. Those habits include sleeping and waking around the same time everyday to maintain a good circadian pattern to sleep. “Collectively, our work has shown that total sleep time is not the key feature – we see no correlation between sleep time and over-sleep memory boost,” she says. “Rather this benefit is specific to certain sleep stages,” and she says that healthy sleep habits are necessary for reaching the deeper form of non-REM sleep, as well as getting sufficient REM.
Looking to the future of sleep and memory research, Spencer is interested in seeing how changes in sleep across a person’s lifespan affects memory over time. She wants to use that information to try to improve learning difficulties in certain populations – for example older adults – by improving sleep. “That’s when all this work begins really paying off is when we can really test the translational message that we tout. I think there’s important groundwork that must be laid to even make predictions regarding those populations but that next step is exciting.”
-Lisa M.P. Munoz
Media contact: Lisa M.P. Munoz, CNS Public Information Officer,