Have you ever struggled to fall asleep?
Most people experience sleeping troubles at some point throughout their life, whether they are current students, night-shift employees, new parents, or simply living humans. Although this problem is nearly universal, few people know why lack of sleep causes them to forget past events or struggle to learn new information the next day. The study “Slow Wave and REM Sleep Deprivation Effects on Explicit and Implicit Memory During Sleep” conducted in 2015 explains why this occurs and how the different sleep cycle stages affect specific types of memory.
The research aimed to discover whether the selective deprivation of slow wave sleep (SWS) or rapid eye movement (REM) sleep would have different effects on the consolidation of one’s explicit and implicit memory (Casey et al. 2016). SWS is considered stage three of four of the sleep cycle and is also called “deep sleep”. REM sleep is the final stage in the sleep cycle when the body experiences muscle atonia – temporary paralysis of the muscles besides the eyes and lungs. SWS occurs mostly in the first part of the night, while REM sleep takes up the majority of the second half of the night (Suni and Vyas 2022).
Prior to this study, dozens of researchers analyzed the stages of the sleep cycle in relation to memory processing, which has led to a wide range of conclusions. These earlier studies mainly used the split-night design in which participants undergo partial sleep deprivation, either in the early or late parts of the night rather than in small increments of each (Fowler et al. 1973; Yaroush et al. 1971). Their data formulated the dual process hypothesis: SWS sleep controls the consolidation of explicit memory, while REM sleep facilitates the consolidation of implicit memory. Explicit memory is generally considered long-term memory that is concerned with the recollection of facts and events; implicit memory encompasses all information that one can remember unconsciously and easily.
Many researchers disagree with the accuracy of these findings, resulting in a new “sequential hypothesis” that proposes “that sleep optimizes the consolidation of a memory when SWS and REM sleep occur in cyclical succession across the night” (Stickgold 2009). This study aims to compare these hypotheses using new technology and a new test design to uncover the true relationship between sleep and memory consolidation.
The experiment used eighteen undergraduate students from King’s College London, who each agreed to the study through a written consent form. Background psychological tests were completed two weeks before the sleep study to determine the participants involved. These participants were between the ages of eighteen and thirty-five years old, right-handed, and fluent English speakers. Furthermore, they did not work night shifts nor have a diagnosed sleep or memory disorder. Two weeks prior to the study, the participants self-recorded their sleeping patterns but were required to sleep between eleven at night until six in the morning at a minimum. According to the journal article “Sleep duration and cognition: Is there an ideal amount”, six and a half hours of sleep is the common balance between too much and too little sleep. Getting much more or much less than this can negatively impair cognitive performance (Mantua and Simonelli 2019).
The design of the study used three independent variables: a control night of undisturbed sleep, SWS deprivation, and REM sleep deprivation. The sleep studies occurred over a period of three nights with a one-week interval between to minimize any interferences. Participants arrived at the sleep laboratory by eight pm to attach and calibrate polysomnography (PSG) electrodes onto them. PSG devices monitor one’s breathing patterns, blood oxygen levels, primary organ activity, and arm and leg movement throughout the night (Armon 2020). Testing began at nine and took approximately one hour before the lights were switched off at eleven. Different participants were awoken at different times throughout the night to interrupt either SWS or REM sleep. When they awoke, each described an autobiographical memory in response to a stimulus word drawn from a list of common nouns. In the morning, all participants were woken at six am and waited twenty minutes before testing, where they repeated the tasks from the morning prior. These tasks were either published alternatives to standardized tasks or new alternatives of experimental tasks that tested one’s visual and verbal recall.
The behavioral findings found that neither SWS nor REM sleep deprivation affected implicit memory. However, SWS deprivation disrupted the consolidation of explicit memories for visuospatial location, while “both SWS and REM sleep deprivation adversely affected explicit verbal recall” (Casey et al. 2016). The visuospatial location of the brain is important for short-term memory and recall, specifically a person’s ability to identify visual and spatial relationships in their surroundings (Baars and Gage 2010). These results emphasize the importance of both SWS and REM sleep each night for explicit memory consolidation. Without a proper amount of sleep, individuals will start to find it more and more difficult to complete simple tasks associated with explicit memory, such as getting dressed, cooking a meal, or making a bed.
The authors admitted to a variety of potential limitations in the study that may affect the reliability of the results. All of the participants were young, healthy, and well-educated, meaning they may have been more resistant to the effects of sleep deprivation. Also, since their sleep recordings before the study were self-reported, they are not completely reliable. Finally, the authors mentioned that selective sleep deprivation does not reflect possible brain dysfunctions, which may disrupt memory consolidation more than lack of SWS and/or REM sleep.
There were three potential flaws in the study beyond what the authors mentioned as well. First, the study consisted of three separate nights, which does not provide enough data to compare two separate, complex hypotheses. Second, eighteen participants is a small sample size, which could affect the accuracy of the data. Third, each participant naturally has a unique tolerance for sleep deprivation and requires differing amounts of sleep per night. The study did not consider this and, instead, marked six hours of sleep as the time necessary for memory consolidation to occur for each participant based on prior research.
Despite these potential flaws and limitations, this study provides useful data by creating a base for understanding the relationship between memory consolidation and the stages of the sleep cycle. Furthermore, it is an ethical study in which the participants signed a detailed consent form that covered the step-by-step process of the experiment, which enhances the study’s credibility.
Although the study provides useful, ethical data, its potential inaccuracy keeps it from being applicable to real-life situations. The results need to be tested further across several additional nights with a variety of unique participants in order to be more reliable. Three nights of research with only eighteen participants, all of whom are young and healthy, is not diverse enough to provide accurate research for a larger population. In the future, the study should intensify and evolve to last at least three months with two days of testing each week in order to accumulate accurate data while also utilizing time. It should include thirty or more participants that vary across a broader range of ages and occupations as well. According to the National Library of Medicine, one’s age and sex significantly impact the effects of sleep deprivation on cognitive performance, including memory consolidation: “during prolonged wakefulness, cognitive performance seems to be maintained better in aging people than in younger ones” and “men typically have … higher visuo-constructive performance, whereas women perform better in visuomotor speed and some verbal functions” (Alhola and Polo-Kantola 2007). These different sleeping and cognitive performance patterns need to be considered during testing.
A more intense strategy would provide accurate data that is useful and applicable for most of the population. More people need to be informed about the importance of completing the sleep cycle each night in order to properly learn and perform basic tasks, remember new information, and properly consolidate other memories.
References
Aloha, P. and Polo-Kantola, P. 2007. Sleep deprivation: Impact on cognitive performance. Neuropsychiatric Disease and Treatment, 3(5):553–567. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2656292/.
Armon, C. 2020. Polysomnography. Medscape. https://emedicine.medscape.com/article/1188764-overview.
Baars, B. J. and Gage, N. M. 2010. Visuospatial sketchpad. Cognition, Brain, and Consciousness (Second Edition), 32-61. https://doi.org/10.1016/B978-0-12-375070-9.00002-4.
Casey, S. J., Solomons, L. C., Steier, J., Kabra, N., Burnside, A., Pengo, M. F., Moxham, J., Goldstein, L. H., and Kopelman, M. D. 2016. Slow wave and REM sleep deprivation effects on explicit and implicit memory during sleep. Neuropsychology, 30(8):931–945. https://doi.org/10.1037/neu0000314.
Fowler, M. J., Sullivan, M. J., and Eckstrand, B. R. 1973. Sleep and memory. Science, 179:302-304. https://pubmed.ncbi.nlm.nih.gov/4345657/.
Mantua, J. and Simonelli, G. 2019. Sleep duration and cognition: Is there an ideal amount? Sleep, 42(3):1-3. https://doi.org/10.1093/sleep/zsz010.
Stickgold, R. (2009). How do I remember? Let me count the ways. Sleep Medicine Review, 13:305-308. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2739268/.
Suni, E. and Vyas, N. 2022. Stages of sleep. The Sleep Foundation. https://www.sleepfoundation.org/stages-of-sleep.
Yaroush, R., Sullivan, M. J., and Echstrand, B. R. 1971. Effect of sleep on memory: II. Differential effect of the first and second half of the night. Journal of Experimental Psychology, 88:361-366. https://pubmed.ncbi.nlm.nih.gov/4326302/.
Featured Image: Streeter, K. The Brain on a Pillow. NPR. https://www.npr.org/sections/health-shots/2013/10/18/236211811/brains-sweep-themselves-clean-of-toxins-during-sleep.