Article In Brief
New research found an increase in left hippocampal activity following daytime naps, which correlated with increased sleep spindles and improved memory encoding performance.
Sleep scientists at the Yong Loo Lin School of Medicine at the National University of Singapore have answered the age-old question about how napping can improve memory encoding: they observed an increase in left hippocampal activity following naps, which correlated on EEG with increased slow, high-amplitude waves—so-called sleep spindles—and improved memory performance.
Michael W. L. Chee, MBBS, professor at the Center for Sleep and Cognition, and his colleague Ju Lynn Ong, MD, MPH, the lead investigator, studied memory performance in healthy adults before and after a 90-minute mid-afternoon nap. (The average napper slept 60 minutes.) They compared performance in those who napped with a control group, who watched a documentary instead of napping.
They looked at how sleep changes the formation of new memories, as opposed to most sleep studies that test how sleep helps strengthen existing memories. Those who napped had a 21 percent better ability to recall the paired list of words than those who stayed awake. Memory performance among those who napped was correlated with a higher sleep spindle count and showed greater hippocampal activation on MRI after napping and performing word pairing and retrieval exercises.
Scientists have explored the science of sleep and napping for decades and have reported similar findings on improvements in memory performance. For example, in 2019, a study published in the Journal of Sleep Research also showed a powerful effect of napping on consolidation of declarative and procedural information. Another study by German investigators published in 2008 in the same journal found that a short daytime nap also improved memory performance.
Study Design, Findings
In the current study, the scientists tested 40 healthy young adults between 18- and 35-years-old who were accepted into the study if they had a habitual nocturnal sleep duration of 6.5 to nine hours. During the first of two visits to the lab, participants filled out sleep and lifestyle questionnaires and practiced paired word-learning tasks. They were all given an actigraph to wear for a week to ensure they kept to their declared sleep timings. Minimally, they had to keep to their normal sleep cycle for at least three days before returning to the lab for the experiment.
Study subjects were randomized to either a 90-minute nap session or time spent watching a documentary. In the first of two encoding-word retrieval sessions, all participants were asked to remember as many paired words as they could while in the MRI scanner. The retrieval sessions were done outside the scanner 40 minutes later. Then, they either napped or watched the documentary. The nappers were monitored with polysomnography. Thirty to 45 minutes after the nap or documentary, the participants did a second-word pair encoding/MRI session, followed by a retrieval session 40 minutes later.
The polysomnography recordings allowed them to measure total sleep time, the number of sleep spindles, the density of the spindles per minute, their duration, and peak frequency. The afternoon nappers encoded significantly more word pairs on average than those who stayed awake during the nap period.
The investigators also observed demonstration of higher hippocampal activity during encoding that followed a nap compared to before it, but no significant difference in the two encoding sessions in those who watched the documentary. Finally, the extent of nap-related memory improvement correlated with the fast spindle count during the naps.
The increase in hippocampal activation during post-nap encoding and the higher fast spindle frequency counts in those showing greater memory improvement are consistent with other experimental findings related to synaptic downscaling, whereby high priority memories and their associated synaptic networks are maintained while connections are removed to free up memory capacity to facilitate new learning.
In related work published by the same investigators in Sleep in 2020, habitual nappers seemed to be more reliant on naps for restoring hippocampal-dependent memory performance. Several other studies from this team using other tasks have also shown that when nocturnal sleep is in short supply, a mid-afternoon nap reliably boosts memory performance in adolescents and young adults.
“This was a very well-designed translational study that furthers our understanding of the link between sleep and augmentation of learning,” said Charlene Gamaldo, MD, FAAN, professor of neurology and psychiatry, and medical director of the John Hopkins Center for Sleep and Wellness.
“Several studies have shown associations with sleep and hippocampal activity to suggest sleep serves as facilitator of memory and learning. However, this study is one of the first to show this in the context of a nap intervention for individuals who are not sleep-deprived. The study also ties hippocampal activity to spindle activity features (density, rate), other sleep physiological biomarkers that have been associated with learning and memory.”
“This study definitely provides additional support for the benefit of napping. What remains are the questions regarding the amount and timing of the naps. For instance, this study allowed a 90-minute nap opportunity whereas, in general clinical practice, patients are recommended to take naps that are usually between 20-40 minutes. Moreover, naps are generally recommended between one and four in the afternoon to align best with the natural circadian ‘siesta’ dip. So it would be interesting to see further studies to help reconcile both the duration and timing of naps in light of these findings.”
Dr. Gamaldo also said that sleep spindles have been speculated to be a marker of the “brain's gateway traffic of information and would seem reasonable in regard to our understanding of the role of the hippocampus in short-term memory that the hippocampus is likely one of the primary destinations for spindle traffic.
“We have known that sleep helps consolidate learning material,” added Alberto Ramos, MD, MSPH, FAAN, associate professor of neurology and research director of the Sleep Disorders Program at the Miller School of Medicine in Miami. “But these scientists have shown that sleep spindles held encode memories and also allows for them to go from the hippocampus to other areas of the cortex, and primes the hippocampus to learn new information once you awake.”
“We advocate sleep, nutrition, and exercise as three critical factors that increase the chances for a long and healthy life,” he added. “Sleep, the timing, and quality, should always be in our discussions with our patients.”
Dr. Ramos's research team studies middle-aged Hispanic adults and identified a link between poor sleep and cognitive decline. Those who took longer to fall asleep at night had lower scores on verbal memory and learning over the course of this longitudinal study, published in December in Alzheimer's & Dementia. They also reported that those habitual nappers had less decline in memory and learning problems. He believes that future studies can help define if seven to nine hours of sleep every day could be a strategy to minimize the risk of dementia. In addition, the big unknown is whether treating common sleep disorders, such as sleep apnea, can lower that risk.
“This is more evidence that timed naps can help improve memory,” said David M. Holtzman, MD, FAAN, the Andrew B. and Gretchen P. Jones professor and chairman of neurology at Washington University School of Medicine. Dr. Holtzman and his colleagues study sleep in older people and have found that more naps are associated with greater Alzheimer's amyloid pathology; these people who were not napping on purpose also naturally fell asleep more during the day, he noted.
“It would be interesting to do a study to see whether habitual napping in adulthood would be protective,” he added. “If you are enhancing memory it is likely to be linked to increased brain plasticity.”
Diego Z. Carvalho, MD, a sleep physician, and researcher at the Mayo Clinic in Rochester, MN, added that good sleep quality is associated with memory consolidation and protects against cognitive decline. The protection against cognitive decline is also associated with the role of sleep in reducing the release and increasing the clearance of toxic metabolic by-products resulting from neuronal activity (for example, p-tau, amyloid-beta). Both daytime napping and nighttime sleep should contribute to these processes.
He explained that “the most accepted hypothesis is that sleep is important for synaptic homeostasis, causing downscaling of synaptic connections that are unnecessary and strengthening of synaptic connections related to important memories. There is a substantial amount of evidence suggesting that initially, the hippocampus binds pieces of memory representations in different parts of the neocortex. During deep sleep, there is a replay of synapses related to encoded memories, with transferring of the information from the hippocampus to the cortex. This process allows long-term memory consolidation in the cortex, independent of the hippocampus.”
“During REM sleep, this process continues with the pruning of synapses providing further downscaling of connections, while important synapses are strengthened,” he added.
There are still many puzzles. In the elderly, daytime napping is associated with a higher risk of cognitive decline and mortality but Dr. Carvalho said “it is likely that napping is not the cause of it, but only a consequence of an underlying problem, which is probably related to untreated sleep disorders and neurodegeneration of wake-promoting and circadian-rhythm centers in the brain.”
“In the end, I always ask patients about how they feel about their naps. If they feel better, more rested, or more alert and they have the opportunity for it, I encourage them to continue to nap, particularly if it is not affecting nighttime sleep and there is no sleep restriction or disorder that requires intervention. Elderly patients who are napping very frequently require further workup to assess for sleep disorders,” said Dr. Carvalho.
“It's an interesting finding and extends some of our thinking in young adults,” added Rebecca M. C. Spencer, PhD, professor of psychological and brain sciences at the University of Massachusetts, Amherst, who studies napping in children. “A 21 percent difference in learned word pairs between the nappers and those who were awake during the same time period is very impressive. The question is: Do these findings extend to tasks of daily living? I don't see why they wouldn't.”
Dr. Spencer said that the need for sleep goes down from young adults into older adults, but “we don't know whether it is going down because older people don't require as much sleep.”