In this blog post, we take an in-depth look at the importance of REM sleep and its effects on memory through various scientific studies and examples.
Sleep is one of the basic necessities for human survival. According to the Korean dictionary, sleep is defined as “a state of unconsciousness with closed eyes.” Although humans spend one-third of their lives sleeping, many people today view sleep as a luxury due to their busy lifestyles. For office workers, late work hours often lead to sleeping late, while students frequently reduce their sleep time due to endless homework and exam preparation, resulting in most people getting far less sleep than what is necessary for good health. For many, this amount of sleep is unrealistic. Since it is possible to function adequately with less sleep, many have become accustomed to a lifestyle with reduced sleep. However, sleep is essential for humans as it provides rest, recharges energy expended during daytime activities, and plays a crucial role in various functions such as immunity and concentration. One of these functions is enhancing memory. While scientists continue to debate the exact relationship between sleep and memory, many researchers argue that REM (Rapid Eye Movement) sleep, a stage of the sleep cycle, is crucial for improving memory. However, simply reaching REM sleep does not guarantee that one will remember everything learned recently, so it may be challenging to claim that REM sleep always helps memory under all circumstances and conditions. Therefore, to examine this claim, it is necessary to understand how REM sleep affects memory.
When we close our eyes and fall asleep, the sleep cycle begins. The sleep cycle consists of approximately five stages, each of which involves various phenomena occurring in the body and brain. The first stage occurs when people first fall asleep. Neural activity becomes asynchronous, and the onset of sleep occurs. At the same time, theta waves—brain waves that primarily appear when drowsiness sets in—are extensively formed in the brain. In the next stage, heart rate and breathing slow down, and brain waves become longer and slower. Additionally, sleep spindles (brain waves with low amplitude and a frequency of 12–14 Hz) begin. In the third stage, deep sleep begins, and breathing, heart rate, and all brain activity slow down further, with neural activity becoming synchronized. The brain then forms large, slow delta waves. Unlike theta waves, delta waves are brain waves that appear when you are in deep sleep and show little movement. In the fourth stage, heart rate, breathing, and brain activity slow down the most. Brain waves become highly synchronized, and neural activity decreases overall. Additionally, the amount of slow delta waves increases.
The fifth stage of the sleep cycle is REM sleep, which is known to be the most beneficial stage for memory. During this stage, the eyes move rapidly back and forth, and dreams become more vivid and frequent. Muscle tension is released, and it is said that the brain stores much of what was learned and studied during the day for long-term memory. Many scientists emphasize the importance of REM sleep.
This is because there are studies and data suggesting that REM sleep is a factor that helps memory during sleep. These results were obtained through animal experiments and demonstrated how important REM sleep is for memory. For example, mice were placed in a maze with food at the end, and they were taught to find their way through the maze’s complex paths to reach the food. Observing this, it was found that the amount of REM sleep increased. Researchers argue that the fact that REM sleep increases when learning new knowledge suggests that REM sleep is important in the process of the brain memorizing new information and consolidating that knowledge. However, these changes in REM sleep may not be directly caused by the association between memory and REM sleep. In repeated experiments, an increase in REM sleep occurred even when the probability of the mice learning less was high, suggesting that other factors may be causing the increase. The stress, pressure, and emotional stimuli from being trapped in a maze and forced to find the exit could have triggered REM sleep. In fact, according to one study, an increase in REM sleep was observed when animals were exposed to an appropriate amount of stress. Therefore, it is difficult to measure and separate stress, emotional factors, and learning, making it challenging to definitively determine what influences REM sleep.
In other animal experiments, specific laboratory mice were used to significantly reduce REM sleep and study its impact on memory. This was based on the hypothesis that if new information is remembered and learned in a state where REM sleep is almost absent, it serves as evidence that REM sleep does not significantly aid memory. In fact, many scientists placed mice on a platform floating in water and observed them. They used the “platform technique,” where the mice’s leg muscles relaxed during REM sleep, causing them to fall into the water and wake up, preventing them from returning to REM sleep. These mice struggled to learn new tasks and exhibited significantly impaired learning abilities compared to normal mice. This was argued to occur because the process of consolidating and memorizing the steps necessary to complete a new task takes place as one reaches the REM sleep stage. However, this phenomenon may not be caused by REM sleep itself. Placing active animals like mice on a platform and restricting their movement can cause severe stress. Falling into cold water can also induce hypothermia, which further increases stress. It has been proven by numerous studies that acute stress can cause a sharp decline in memory by secreting the hormone cortisol. Mice injected with cortisol took longer to learn a maze route compared to normal mice, made more mistakes, and were less likely to reach the end.
Therefore, in such cases, it is reasonable to assume that stress may have caused more harm to memory than REM sleep. Animal experiments have limitations, and there is no certainty that the results can be applied to humans. Additionally, the amount of REM sleep varies among animals, making it inappropriate to compare REM sleep in mice and humans. Therefore, it is necessary to review research findings from human experiments that demonstrate the relationship between REM sleep and memory.
Similar to animal experiments, studies have investigated whether the amount of REM sleep increases when people are given tasks requiring the memorization of new information. According to Smith and Lapp’s study, observing students’ sleep cycles after an exam period revealed that while the duration of REM sleep did not increase, its density did. These results refute the claim that stress increases the amount of REM sleep and suggest that REM sleep may be important for the memorization required for exam preparation. However, the control group used in this experiment consisted of students who did not take the exam due to personal circumstances. Since there was no significant difference in the amount of new information learned and memorized between these students and those who took the exam, and since it could not be confirmed that stress or other factors that could disrupt sleep were consistent among all students, the reliability of the results may be limited. If REM sleep were eliminated in humans, as it has been in animal experiments, what effect would this have on memory? In cases where brain damage occurs in the brainstem due to illness or injury, this is theoretically equivalent to the removal of REM sleep. However, according to research, there is no clear evidence or reported cases indicating that individuals without this part of the brain have impaired or lost the ability to form new memories. This contradicts the claim that REM sleep aids in the formation and integration of memories and can be cited as evidence that there is no relationship between the two.
Applying this to everyday life, many middle and high school students, as well as college students, often have to stay up all night studying for exams. In such cases, they may not get any REM sleep or it may be disrupted. From personal experience, I have often found that by repeatedly thinking about the material I studied in my head, I was able to recall it slowly without going through the REM sleep stage and eventually remembered everything, allowing me to perform well on exams. In other words, it is impossible to rule out the possibility that memory impairment due to lack of sleep is a result of factors such as reduced concentration or stress. Additionally, the amount of sleep or REM sleep required varies from person to person due to genetic factors, and it can also differ depending on an individual’s upbringing and environment. Some students may have consistently slept insufficiently for several years, leading to the question of whether the brain becomes accustomed to this state. It is also unclear whether individuals accustomed to sleep deprivation experience less memory impairment when deprived of REM sleep compared to those who typically undergo REM sleep. If such a phenomenon exists, further research is needed to determine whether prolonged exposure to REM sleep can reverse this effect.
Sleep is extremely important for humans for many reasons, and memory is one of them. It is widely known that sufficient sleep is necessary for optimal memory, and there are many reasons behind this phenomenon. While many scientists emphasize the importance of REM sleep, this remains a subject of debate. Therefore, it may be challenging to unconditionally accept REM sleep as a crucial factor for memory based solely on current research findings. Additionally, there is conflicting evidence suggesting that REM sleep may not be as important as some scientists claim. As a result, the significance of REM sleep’s impact on memory remains unclear, and further research and study are necessary.
