An Independent Project by Monika Nemcova ’19
Part 3 of 4
In my Spanish class, one of the girls summarized the universal student’s dilemma: we can either study, have a social life, or sleep – and as our time is not infinite, we manage to do only two of the three options. Well, most of us do not consider eliminating friends and becoming a hermit a viable life option. And at Andover, studying is really not optional – which leaves sleep as the least important activity. Nights thus become an endless reservoir of time which is spent either by writing that English essay or Snapchating with a well-meaning friend. For my whole year at PA, I have never met someone saying that getting a good night of sleep was their priority. Yet, as I try to argue in my series of posts about the importance of sleep, sleeping well is one of the biggest factors that influence our behavior and our performance in life. In this article, I would like to show you that the distinct choice between studying and sleeping does not exist – as the latter directly enables the former. In order to learn, the brain simply requires sleep.
When thinking about the role of sleep in learning, I remembered my dad’s childhood story that has never ceased to fascinate me. Back in the seventies, the Czech Republic still had a communist government, that made Russian a compulsory subject in all schools (as a nod to the USSR, which backed the regime). My dad, never one for learning languages, hated his Russian lessons with a passion. When he was in sixth grade, his Russian teacher insisted on learning long, lyric poetry by heart as a part of the curriculum. No one expected much of my dad. But he surprised everyone by a fluent recitation. He explained to me that he had found out that if he studied the poem straight before going to sleep, he then slept very poorly but dreamt about the poem. In the morning, he could recall it without problems. In fact, as he was telling me the story, he was still able to recite the poem, thirty years after.
The story of my dad might be an extreme case, but the relationship between dreaming about a task and improvement in that activity has been experimentally established . In 2010, researchers taught two groups of people how to navigate a 3D maze. After the learning session, one of the groups went to sleep and the second one remained awake. In the napping group, the subjects who reported dreaming about things associated with the maze fared significantly better in the re-testing of the navigation. In contrast, no such distinction appeared in the awake group – the performance of the subjects was not affected by thinking about the maze. The researchers were careful not to generalize the findings as direct causation between dream experience and memory consolidation. Instead, they proposed that both the dreaming and the improvement of the task were the results of the processes of “memory reactivation and consolidation in sleep” . So dreaming about the task would be just a side product of an efficiently learning brain.
However, that still does not explain how do these “processes” function, nor, for that matter, why was my dad’s learning technique effective only when he studied directly before going to bed. In order to understand that, we need to know a bit about how learning functions. The neurons in our brains are connected in intricate nets and pathways – one neuron always receives inputs from many others and based on the type and strength of the signal it receives, it either fires as well or stays silent. However, this system is not static. Both external (seeing a member of your tribe get eaten by a tiger because he was too loud) and internal (feeling pain after trying to touch fire) stimuli can affect the strength between the individual neurons. So in my first case, the connection between the neural pathway responsible for being stealthy and the pathway recognizing tiger territories strengthens. In the second case, the pathways for “great things to touch” and “fire” become less connected. It is not that easy but that is the basic principle. When the connection between two neurons strengthens, the neuroscientists say that long-term potentiation (LTP) has occurred. Weakening connection is called depotentiation. The last thing we need to know about learning is that the strengthening of the neural connections can be either short-term or long-term. A new experience evokes a short-term change, which, if it were left so, would disappear after four to six hours . The brain needs to further strengthen the connection to make it last. And that is where sleep comes in.
Last remainder: LTP and depotentiation are chemical processes and as such depend on the brain’s own levels of chemicals, called neurotransmitters. I talked about neurotransmitters at length last time, so just a quick review relevant to learning. In REM sleep, the most prominent neurotransmitter is acetylcholine and the levels of other chemicals are low (for our context are important norepinephrine and serotonin). In NREM sleep, acetylcholine is low and both norepinephrine and serotonin occur at moderate to low levels. The neurotransmitters occurring in the brain in different sleep phases determine whatever and how the synapses can be strengthened .
It has been experimentally established that when we learn something new, the proportion of REM in our overall sleep increases. This effect appears only when we actually learn and improve, not just all the time when we try to learn . When we master that task, the amount of REM falls to normal . That alone would be a good indicator that REM sleep somehow contributes to the learning process. However, there is also another proof in favor of that theory – the presence of the before-mentioned neurotransmitter acetylcholine. Acetylcholine makes the synapses (connections between neurons) especially plastic, which allows them to undergo either LTP or depotentiation .
LTP occurs when the neuron at the synapse fires in accordance with the general firing pattern of the brain – that EEG diagram pattern which I talked about last time (during REM, the firing pattern looks very much like the pattern of wakefulness). Depotentiation, a process as equally important for learning as LTP, is the result of a neuron firing in-between the pattern seen on the EEG. Also, depotentiation appears only in the absence of both norepinephrine and serotonin – which, again, makes the REM phase of sleep ideal for this process, as the levels of these neurotransmitters are at their lowest during REM [2, 3].
We have known for a long time that NREM sleep is also important for memory consolidation – the neurons which fired during the learning process were recorded firing again in the same order, albeit in an accelerated pattern (up to 300x). However, there is one obvious obstacle to proposing that NREM sleep also contributes to LTP – namely the absence of acetylcholine. Without acetylcholine, the short-term LTP potentiation, which happens during REM, cannot be initiated. However, it appears that NREM sleep allows the short-term LTP previously established during the REM phase to be consolidated into the more permanent long-term LTP. In favor of this hypothesis speaks the fact that protein synthesis necessary for long-term LTP notably accelerates during the NREM .
In addition, NREM sleep seems to contribute to depotentiation of the unused or even obstructive synapses .
Now we can finally determine the real-life consequences of insufficient sleep. Because the REM phase is the only time when brain has high levels of acetylcholine and simultaneously low levels of other, obstructive neurotransmitters, it is the only time when LTP can be initiated fully. Experiments determined that subjects that were sleep deprived after a learning session showed no performance improvement following day . That is one thing to consider – if we do not get enough REM sleep, learning simply does not occur. Going to sleep might thus be the most effective thing to do before a test, instead of trying to revise the material one more time. Also, subjects allowed to go to sleep immediately after a learning session had especially enhanced performance  – which probably explains my childhood mystery concerning my dad’s miraculous technique of poetry memorization. So I hope that the relationship between sleep and learning is less mysterious now (also, do not trust it when someone claims to learn something new while sleeping – it is a myth originating in faulty data analysis during the early twentieth century and as such was debunked in the 50’s ). Next time, I would like to focus on circadian rhythms and how do they affect both our sleep and our daily routines.
 Wamsley, E. J., Tucker, M., Payne, J. D., Benavides, J., & Stickgold, R. (2010). Dreaming of a Learning Task is Associated with Enhanced Sleep-Dependent Memory Consolidation. Current Biology, 20(9), 850-855. https://doi.org/10.1016/j.cub.2010.03.027
 Poe, G. R., Walsh, C. M., & Bjorness, T. E. (2010). Cognitive Neuroscience of Sleep. Progress in Brain Research, 185, 1-19. https://doi.org/10.1016/B978-0-444-53702-7.00001-4
 Maquet, P. (2001). The Role of Sleep in Learning and Memory. Science, 294(5544), 1048-1052. https://doi.org/10.1126/science.1062856
 Kang, S. (2018, October 28). Can you learn in your sleep? [Blog post]. Retrieved from Brainy Sundays website: https://scanberlin.com/2018/10/28/can-you-learn-in-your-sleep/