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What does sleep do for our memory?

Anya Singh

Introduction

Memory is understood as the persistence of learning and retention of information over time, and it is crucial to our daily lives; being able to store, retain, and retrieve memories gives us the ability to reflect on our past, make plans for our future, and more. As such, extensive research has been done to identify factors that improve our memory, which include regular exercise, appropriate nutrition, and adequate sleep, among many others (Harvard Medical School). 

Sleep stands out among these factors—according to the U.S. Department of Health and Human Services (2023), us humans spend a third of our lives sleeping! Because of this, sleep is also well understood to have a large impact on our wellbeing. While we’re asleep, we give our brains time to rest, remove toxins, and prepare for the next day, and a lack of quality sleep can greatly increase our risk of diseases and disorders, ranging from heart diseases to memory disorders such as dementia (Wein, 2021). But what exactly goes on during sleep that has such an impact on our memory? 

Neurological Standpoint

A Brief Overview of Memory

How do we form memories in the first place? The formation of a memory, or the encoding of an experience, involves taking in visual and auditory information from our environment. Upon receiving this input, a particular set of neurons in our brain is prompted to fire, or to become active (The University of Queensland, 2018). The concurrent activation of the neurons in this group comprises a “neuronal representation” of the event in our brains and is the basis of memory formation. 

This process is effective in continuously encoding information in our hippocampus, a specific site in the brain dedicated to memory consolidation. However, it results in the formation of memories which last for short periods of time and which are especially vulnerable to interference from other newly encoded information. The recall of these memories can help stabilize them, though; each time a previously encoded experience or bit of information is recalled, the set of neurons that fired when the information was first encoded—the neuronal representation—fires again, or reactivates, and the connection between these neurons is strengthened. This persistent strengthening of connections between neurons is what helps a memory last longer.

Even with the mechanism of strengthening memories through recall, we unfortunately still run into two issues when it comes to meaningful memory:

  1. Limited storage in the hippocampus

As referenced earlier, the hippocampus is the structure in our brains that initially stores all of our newly created memories—but its storage space is finite. As we continue to take in stimuli from our environment and form new memories, the hippocampus runs out of space to store this information. The encoding of new, potentially conflicting, information has the capacity to erase older memories in the hippocampus due to its limited space—something that we formally call “catastrophic interference” (Rasch & Born, 2013). 

2. Memory Strength

The prospect of catastrophic interference shows how freshly formed memories aren’t incredibly resistant to interference from memories of other events and experiences and are relatively temporary as a result. While a cascade of events on the molecular level works to stabilize memories shortly after an initial encoding and each subsequent recall, more is needed to truly strengthen our memories and help them remain intact in the long-term.

Systems Consolidation 

Luckily, a process we call “systems consolidation” is able to address these issues! 

Systems consolidation consists essentially of interactions between the hippocampus and another brain structure called the neocortex, a region of the brain that is dedicated to higher-level functions such as sensory perception, cognition, and goal-directed behavior (ScienceDirect, n.d.). During sleep, the set of neurons that fired together when a memory was first encoded—the neuronal representation of a memory—is repeatedly activated along with the neocortical brain regions that were once involved with the encoding of the memory. Just as connections between neurons get strengthened through recall as described earlier, as these reactivations continue, the neuronal representation of the memory gets strengthened specifically in the neocortex. As this process moves along, the neuronal representation slowly starts to wane its dependence on the hippocampus and becomes integrated with the neocortex. Eventually, the representation is no longer stored in the hippocampus and is able to be retrieved using the neocortex only (Klinzing et al., 2019). 

When our brains go through this process, the limited storage of the hippocampus becomes less of an issue—systems consolidation frees up space in the hippocampus by allowing memories to rely on the neocortex instead! Our brains are also given the extra push to stabilize memories for long-term storage with systems consolidation, as the process truly strengthens the neuronal connections and gives them shelter from constant interference. Not only does systems consolidation address our two issues, but because the process involves strengthening of some neural connections and the weakening of others, as well as integrating memories into related ones and into existing bodies of knowledge, it also serves to organize our experiences and enhance our levels of abstraction (Klinzing et al., 2019). 

Why Sleep?

If the interactions that take place during systems consolidation are what give rise to these benefits in our memory, what exactly is sleep’s role? To tie it all together, sleep is so crucial to our memory as it presents our brains with a window of opportunity to perform this process; during our sleeping hours, we experienced an increased amount of neurotransmitters which facilitate communication between the hippocampus and neocortex, as well as a reduced amount of stimulation and lack of new events which helps prevent interference (Genzel & Wixted, n.d.; Yale School of Medicine, 2022). These conditions allow for effective systems consolidation!

Conclusion

Beyond the initial formation of our memories, the strengthening and prolonged storage of them depends on a myriad of processes that occur on both the cellular and neural level—systems consolidation is a particularly important process that works throughout our entire lives to facilitate the stabilization and organization of our memories for the long-term, and it relies on the quality conditions that present themselves during sleep. After understanding what your brain does for you during sleep to ensure that these processes occur to get you ready for a new day of taking in information and creating meaningful memories, hopefully you are more motivated to get more quality sleep!


References

Genzel, L., & Wixted, J. T. (n.d.). Cellular and systems consolidation of declarative memory. http://wixtedlab.ucsd.edu/publications/wixted2017/Genzel_Wixted_2017.pdf 


Harvard Medical School. (n.d.). Memory. Harvard Health. https://www.health.harvard.edu/topics/memory   


Klinzing, J. G., Niethard, N., & Born, J. (2019). Mechanisms of systems memory consolidation during sleep. Nature Neuroscience, 22(10), 1598–1610. https://doi.org/10.1038/s41593-019-0467-3 


Rasch, B., & Born, J. (2013). About Sleep’s Role in Memory. Physiological Reviews, 93(2), 681–766. https://doi.org/10.1152/physrev.00032.2012  



The University of Queensland. (2018, July 23). How are memories formed?. Queensland Brain Institute - University of Queensland. https://qbi.uq.edu.au/brain-basics/memory/how-are-memories-formed   


Wein, H. (Ed.). (2021, April 18). Good Sleep for Good Health. National Institutes of Health. https://newsinhealth.nih.gov/2021/04/good-sleep-good-health   


Yale School of Medicine. (2022, May 10). Sleep’s Crucial Role in Preserving Memory. Yale School of Medicine. https://medicine.yale.edu/news-article/sleeps-crucial-role-in-preserving-memory/   

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