Brainwaves during sleep strengthen memories: Study

Brainwaves produced during sleep helps us store new information in our memory, according to a study that explains how bedtime helps boost our learning.

Researchers have known about the close relationship between sleep and memory for decades. The study, published in the journal NeuroImage, shows how learned information turns into reliable memories during sleep. Researchers from Concordia University in Canada and University of Liege in Belgium studied how declarative information like facts and faces get stored after they have been learned.

Brainwaves specifically, ones called sleep spindles, are fast bursts of electrical activity produced by neurons mainly during Stage 2 sleep, prior to deep sleep. Using medical imaging machines, researchers were able to assess brain activity related to these waves. "It's hypothesised that sleep spindles play an important role in transferring information from the hippocampus to the neo-cortex," said Thanh Dang-Vu, associate professor at Concordia University.

"This has the effect of increasing the strength of memories," Dang-Vu said in a statement. To get the images they needed, the team used both electroencephalography (EEG) and functional magnetic resonance imaging (fMRI). They applied these tools to a group of student volunteers during and after a lab-based face-sequencing task. The students were shown a series of faces and asked to identify the order in which they were shown. The researchers scanned them while they were learning the faces, while they were asleep and again when they woke up and had to recall the sequences. They then came back every day for a week and repeated the task without being scanned. After a week had elapsed, they had memorised the task, and were once again scanned during sleep and asked to recall the sequences. "Our aim was to compare the sleep spindles from the night where the subjects learned the new information to the night where they didn't have any new information to learn but were exposed to the same stimulus with the same faces," Dang-Vu said.

The researchers found that during spindles of the learning night, the regions of the brain that were instrumental in processing faces were reactivated. They also observed that the regions in the brain involved in memory -- especially the hippocampus -- were more active during spindles in the subjects who remembered the task better after sleep.

This reactivation during sleep spindles of the regions involved in learning and memory "falls in line with the theory that during sleep, you are strengthening memories by transferring information from the hippocampus to the regions of the cortex that are important for the consolidation of that specific type of information," Dang-Vu said. Using non-invasive imaging to identify the mechanisms that strengthen memories can lead to improvements in our understanding of how memories work -- and can lead to improved interventions for people with sleep or memory issues.