Imagine this: It’s a Thursday night, you are at the campus library trying to cram in last-minute studying for your upcoming Biochemistry exam. You’re stressed, trying to memorize all steps of glycolysis, and running on 5 hours of sleep. You’ve finally got it down and decide to go home and get some rest. Have you ever wondered how your brain manages to retain all this information? How do you wake up the next day remembering it all? Our neural correlates within the brain are so intricately made and connected, that they can consolidate so many of our memories. Here, we will specifically look at the neural correlates of stress and sleep.
Since many of our most memorable experiences tend to be those that are associated with either extremely pleasurable or unpleasurable situations, it would make sense that stress certainly affects the memory process. Acute stress is found to play a large role within the hippocampus, amygdala, and prefrontal cortex–structures critically involved in memory. Stress increases levels of specific neuromodulators, such as Norepinephrine, that are critically involved in the processing of emotionally salient materials. For example, Norepinephrine elevates amygdala activation, which in turn results in a greater hippocampal response, which is associated with better memory. Glucocorticoids also play a similar role alongside Norepinephrine. The network of the amygdala, hippocampus, and medial prefrontal cortex increases in response to stress and affects memory consolidation within that timeframe. Whether or not this surprises you, it still shows the outstanding capabilities of our body and brain to take ‘negative’ emotions (i.e., stress) and use that to benefit us greatly–think “fight or flight.”
Not only does stress play a role, but sleep can play a role WITH stress to help consolidate important information. Stress has appeared to exaggerate the effects of memory consolidation during sleep. The same network discussed above has shown increased functional connectivity to retrieve information post-sleep. If our student from our example were to deprive themselves of sleep for one night, or even two nights to stay up studying, the functional connection between the network decreases, thus decreasing selective memory consolidation. That is why it is common advice to get a good’s night sleep before any important day–among other benefits, of course.
It is also important to recognize that our sleep cycles have multiple stages, moving from slow-wave sleep (SWS) and rapid eye movements (REM). Slow-wave sleep is the deepest phase of non-REM sleep and is often associated with long-term memory consolidation. By studying the electrophysiological properties of sleep, the study can observe different connections. It is found that SWS facilitates neural connections from the hippocampus to the cortices of the brain. However, REM also plays an important role in the selective consolidation of emotionally relevant memories. There is increased amygdala activity and activity within the network of brain structures. REM theta oscillations (brain waves) are connected to an increase in synaptic plasticity, thus enhancing memory performance. Therefore, there is a complementary relationship between SWS and REM on memory consolidation. Stress plays a role in sleep as it can affect the neurochemical environment, increasing cortisol levels that the research measured, which affected REM theta activity, thus enhancing memory consolidation. Stress can also allow dendritic spine formation.
Although there are many details to this, it is important to understand the larger picture that stress causes different chemicals to be released (norepinephrine, cortisol, dopamine, etc.), which impacts our sleep and more importantly how our memory is consolidated. Based on this evidence, this can also apply whether we’ve had a “good night’s sleep” or not, as long as your brain is able to go through the stages of the sleep cycle discussed here. Stress and sleep coming together allow us to form stronger lost-lasting memory, hopefully helping us remember and do better on anything important the next day!
References
Grella, Stephanie L, et al. “Odor modulates the temporal dynamics of fear memory consolidation,” Learning Memory, 2020, doi: 10.1101/lm.050690.119. PMID: 32179657; PMCID: PMC7079569.
Kim, Sara Y, and Jessica D Payne. “Neural Correlates of Sleep, Stress, and Selective Memory Consolidation.” Current Opinion in Behavioral Sciences, vol. 33, 2020, pp. 57–64., doi:10.1016/j.cobeha.2019.12.009.
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