The study is the first to show that autonomic nervous system activity during REM sleep is linked to memory consolidation.
Such was the finding of a study from the University of California-Riverside (UCR), published in the Proceedings of the National Academy of Sciences.
The study shows that increase in activity of the autonomic nervous system (ANS) during sleep is strongly linked to memory improvement.
The ANS is the part of the nervous system responsible for body functions that are not controlled consciously, such as heartbeat, breathing, and digestion.
Senior author Sara C. Mednick, a professor of psychology at UCR, explains the background to the study:
"Sleep has been shown to facilitate the transformation of recent experiences into long-term, stable memories. But, past studies produced contradictory evidence about which specific sleep features enhance memory performance."
She and her colleagues concluded this meant something as yet unidentified is going on during sleep to promote memory consolidation.
They hit on the idea of the ANS because there is evidence that memory during waking hours improves when the ANS is active. As such, they devised a study to test whether ANS activity during sleep could be the missing link that explains how sleep helps memory consolidation.
Remote Associates Test
For their study, the researchers invited volunteers to take part in a series of tests based on the Remote Associates Test (RAT).
The RAT was devised as a test of creativity. Each item offers three words, and the participant is required to give the answer as a fourth word that links the three words together. For example, an item might offer the words "duck, fold, dollar," for which the correct fourth word would be "bill".
The researchers adapted the RAT for their study by adding a memory component. The participants took the test twice, some taking naps in between, during which time the researchers measured their sleep quality and heart rate variability (a way of monitoring ANS activity).
In the first round of tests, 81 participants were presented with RAT problems. Some were also asked to complete an analogy task that required them to give one-word answers.
Unknown to them, the one-word answers on the analogy task were also "primes" for solving some of the RAT problems they would be asked to answer in the second round.
After the first round, 60 of the participants were invited to take a 90-minute nap, while the rest - the controls - watched a video.
ANS activity in REM sleep linked to better memory
Later in the same day, all participants completed a second round of RAT problems. In some of these problems, the answers were the "prime" words from the analogy task completed in the first session.
The results showed that of the participants who were "primed" with words in the first session, it was the ones who took a nap who were most likely to solve the RAT problems. The controls, who did not take a nap, performed less well.
This result is in line with previous studies and shows that sleep improved memory performance. In fact, around 40 percent of the performance improvement due to the nap could be predicted by the amount of rapid eye movement (REM) sleep, note the researchers.
However, the important finding was revealed when they assessed the link between ANS activity during REM sleep (as measured by heart rate variability): this showed that heart rate variability during REM sleep accounted for up to 73 percent of the memory performance improvement.
"The findings suggest that ANS activity during REM sleep may be an unexplored contributor to sleep-related improvements in memory performance."
Prof. Sara C. Mednick
The researchers suggest the findings will help improve our knowledge of the links between sleep, heart health, and cognitive functioning, as well as further our understanding of the mind-body connection.