You may have heard that sleep is very important during a test or exam week. This is because sleep is important for your memory. Sleep this has been a frequently discussed topic on Brainmatters.nl. Read here why sleep is an essential part of a good learning strategy.
Research in mice
In the US, researchers had athletic mice train to run forward on a spinning cylinder. Before, during, and after this training session, dendrites in the motor cortex were mapped with a special microscope. After the training session, half of the mice were kept awake for 7 hours, while the other half could take a nap. During these 7 hours, the brains of the sleepyheads were monitored with EEG. This allowed the researchers to observe from brainwaves whether the mice were asleep, and what stages of sleep they were in. Two sleep phases can be distinguished: Rapid Eye Movement (REM) sleep and non-REM sleep. During REM sleep, there is great brain activity, our muscles are completely relaxed and we dream often.
It was found that mice that had slept well, formed more new dendritic spines than sleep-deprived mice. This lag in formation of dendritic spines could not be rectified by a longer training session, nor sleep after sleep deprivation. It was also observed that fewer newly formed dendritic spines were preserved in sleep-deprived mice a day later! These mice showed a smaller improvement in their performance on the cylinder after 1 to 5 days than the mice that had slept well. It was also found that non-REM sleep is the phase important for the formation of new dendritic spines. Mice that were kept awake only during REM sleep did not lag behind mice that had slept well. Finally, the researchers saw that only the cells that greatly increased in activity during running were reactivated during non-REM sleep. This reactivation of task-specific cells is probably the process by which new dendritic spines are formed.
What about humans?
This study in mice showed that non-REM sleep in particular, is important for forming new connections in the brain. But we knew less about this non-REM sleep than about REM sleep. What is special: during non-REM sleep, we occasionally see so-called 'sleep spindles' in EEG signals in humans. These are moments when neurons suddenly, and for a very short time, become incredibly active. The usefulness of these spindles was long unclear.
Researchers from England asked subjects to gain knowledge about a certain concept over a number of weeks, for instance, properties of different types of cells. In this way, the subjects developed a so-called 'schema': a kind of coat rack of background knowledge. Such a schema is known to make it easier to learn new facts related to that background knowledge. The researchers were now curious to see to what extent sleep spindles had anything to do with integrating new information with such a schema. To test this, participants were given new information to learn at the end of the learning period, information that was or was not related to the background knowledge. The subjects then slept in the lab overnight while EEG was measured. Finally, it was tested how much of the new information they had remembered. The result: the more powerful and intense the sleep spindles had been during sleep, the better the information related to the schema was remembered. In other words, sleep spindles are directly related to integrating new information with existing background knowledge.
Another British study suggests that newly learned information is reactivated and reinforced in memory during sleep. Participants in this study were asked to learn associations between words and images of objects and environments. The subjects were then allowed to take a nap in the laboratory. During sleep, half of the learned words were played through headphones. With this, the researchers hoped to activate memory traces to see if these word-picture associations were better stored. After taking a nap, the volunteers got back to work. They were shown all the words again and had to indicate which object and environment pictures belonged to each word. This showed that memories were better for associations with the words presented during sleep! During the sleep period, the researchers measured the participants' brain waves via EEG. This showed that sleep spindles emerged when the memories were activated. The researchers were able to distinguish the brain waves associated with object memories and environmental memories. Apparently, these spindles produce unique memory codes for certain activated memories, which probably accounts for the enhanced storage of these memories during sleep.
Can you also enhance these effects of sleep on memory?
We now know that sleep spindles are important for storing learned information and that we can enhance this by activating this information during sleep. But previously, another way to enhance memory storage during sleep was found.
As in the other studies, this experiment measured brain activity in sleeping subjects by EEG. Every time slow brain waves (a different type of activity than spindles) were seen in the EEG pattern, the sleeping subjects were given two short clicks. These clicks fell exactly in the rhythm of the EEG waves and caused the slow rhythm to be amplified. These clicks seemed to have an effect on memory. All subjects had memorised a series of word pairs before going to bed. After the night's sleep, they were asked which pairs they could remember. As expected, the performance of all subjects improved after a night's sleep. But the rhythmic clicks had an additional effect on memory, compared to nights when no clicks were played or when the clicks were not in rhythm.
So it is the timing of the clicks relative to the rhythm of brain waves that is crucial. Therefore, it makes no sense to put a metronome next to the bed just before a French word exam. Unless you enlist some handy neuroscientists who connect this metronome to an EEG device… Taken together, it seems that it makes perfect sense to get a good night's sleep after learning. Your brain cells will then have a chance to activate all the material learned one more time and store it safely. So, for the next exam, no hot steaming cup of coffee, but a nice soft bed.
This article is based on previously published brianmatters articles from 9-6-2015 " Slapen hoort bij een goede leerstrategie” written by Iris van Sambeek, 18-4-2016 "Slaap en Geheugen" by Sander van Bree, 20-3-2018 " Herinneringen uitlezen tijdens slaap " by Job van den Hurk and 29-5-2013 "Klikjes tijdens slaap verbeteren geheugen" written by Bart Aben.
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