The Effects Of The Lack Of Sleep On Memory Consolidation
We spend one-third of our lives sleeping. The importance of sleep was evident in Marie de Manacéine’s experiment in 1894. She kept puppies in constant activity with no sleep, which resulted in their death after a few days. Humans also suffer from sleep deprivation for prolonged periods of time, with effects ranging from a decrease in mental functioning to a distorted sense of time. Just recently in a Hong Kong newspaper, the South China Morning Post, the founder of Huffington Post told Hong Kong citizens that a good night’s sleep was the key to success. Despite knowing the importance of sleep, humans know little about the specific mechanisms through which sleep supports our well-being. In addition to filling this gap in scientific knowledge, a better understanding of why sleep is important and what are the most important functions may give us ways to shape public health initiatives and improve our well-being by encouraging more sleep and teaching ways to compensate for health problems caused by a lack of sleep. Many say sleep is most crucial to learning and forming long-term memories, whereas others either deny that memory is a function of sleep or claim that other functions are more important. This paper will introduce key terms and theories regarding sleep, memory, and memory consolidation; analyze experiments and arguments; and conclude that memory is indeed an important function but not necessarily the most important function of sleep.
In order to analyze whether memory is the most important function of sleep, we first have to understand sleep and memory consolidation by themselves and then the relationship between the two. Sleep consists of non-rapid eye movement sleep (NREM) and rapid eye movement sleep (REM). NREM and REM sleep alternate in an 8-hour sleep period with 4-5 cycles each lasting 90-110 minutes. The typical sleep requirement per night for adults is 7.5-9 hours with 20-25% of deep non-REM sleep, 20-25% of REM sleep and 50% of light non-REM sleep. Memory is the ability to encode, store and recall information, past experiences and habits in the human brain. Memory enables one to adapt its behavior to the ever-changing demands of an environment. There are three stages of memory: encoding, consolidation and retrieval. Encoding is the formation of a new memory trace, consolidation is stabilizing and integrating the labile memory trace into preexisting knowledge networks, and lastly, retrieval is the process of recalling the stored memory. In neurological terms, consolidation is the re-creation of past experiences through synchronous firing of the neurons involved in the original experience. This paper will focus on memory consolidation – the process of changing an initially unstable representation into one that is sturdier and less susceptible to interference.
Initially, it was theorized that sleep plays a passive role in enhancing memories. However, scientists today believe that sleep plays an active role. When we are awake, the brain is optimized for acute processing of external stimuli, encoding new information and retrieving memories. When asleep, the brain provides optimal conditions for consolidation processes, integrating newly encoded memories into long-term storage. One possible theory to explain this is that encoding and consolidation are mutually exclusive and compete for the same neural resources. Hence, the brain is only able to consolidate trace memories when there is no incoming external information to be encoded. Both behavioural and neural evidence suggest that memory consolidation is the most important function of sleep. Katya Potkin and William Bunney performed an experiment to investigate the effects of sleep on auditory declarative memory in adolescents. Declarative memory is the memory of conscious events such as news stories, as opposed to memories gained from experiences that are unconsciously applied to everyday life, such as riding a bike. The subjects of this study were 40 male and female, ages 10-14. They were trained to perform declarative tasks at 9am and tested at 9pm, without sleep in between the intervals. The same subjects were then trained again at 9pm and tested at 9am after sleeping. Results showed an increase of 20.6% in declarative memory when subjects slept (Potkin and Bunney). These findings demonstrate that sleep consolidates memory. But how is memory consolidated? Although our brains dissociate from sensory inputs during sleep, they remain active. Models show the presence of electrical activity in the form of sharp-wave ripples in the hippocampus and large-amplitude slow oscillations in the cortex during slow-wave sleep. The hippocampus is a small organ that supports memory function.
Several studies suggest that memories are consolidated through the interaction between high-frequency sharp-wave ripples and low-frequency slow oscillations, and the re-firing of neural cells to create a “replay” of memories. Place cells are neurons within the hippocampus that have spatial receptive fields. They fire when an animal moves to a specific location. As different place cells are assigned to different regions in space, they provide a cognitive map.
There is convergent evidence that prove the presence of a place cells “replay”. Lee and Wilson recoded place cells of rats that ran on a track. During sleep, the same place cells recapitulate past routes that the animal made previously when they were running. Similarly, another experiment by György Buzsáki and others had rats move around in a maze. When the rats were in slow-wave sleep, a replay of place cells in the same order fired previously refired. Therefore, such “replay” of firing sequences exists and is proposed to be a mechanism for memory consolidation. This theory has been tested by blocking such “replays”, which demonstrated a negative impact on memory. Buzsáki G and others selectively eliminated sharp wave ripples after training spatial memory task periods, which resulted in an impairment in the performance of rats. To further investigate the importance of hippocampal sharp-wave ripples in transferring selective memories into the cortex, a study was conducted by UC Riverside researchers. Using a computational model, the researchers showed that patterns of slow oscillations in the cortex (which their model spontaneously generated) are influenced by sharp-wave ripples in the hippocampus. Sharp-wave ripples determine the spatial pattern of slow oscillations, and these slow oscillation patterns determine the synaptic changes in the cortex. Synaptic changes in return affects the patterns of slow oscillations, which promotes the re-firing of cortical neuron sequences – a replay of a specific memory. Hence, the input of the hippocampus activates memories during slow-wave sleep, causing a “replay”. Results show that these patterns is still present even without further input from the hippocampus to the cortex, suggesting that specific memories are consolidated during slow-wave sleep, where memory traces are formed and becomes independent from the hippocampus. This indicates the importance of sharp-wave ripples during sleep in transferring memory to the cortex. However, the phenomenon of re-firing cells has only been demonstrated in rats since there are very few methods of investigating the behaviour of single cells in humans. Intracranial cell recording is an option, but this can only be executed during brain surgery, which means the brain would have some sort of pathology. On the other hand, intracranial cell recordings in rats are easier to capture because there are fewer ethical considerations in testing rats than humans. Hence, neural replay is a putative neural mechanism of memory consolidation which occurs during sleep, and therefore provides some evidence for the hypothesis that memory consolidation is a very important function of sleep. However, since most studies have been conducted on rats, more research into humans’ ability to connect hippocampal replay to observable behavioural memory needs to be done in order to confirm the hypothesis.
Scientists often disagree that memory consolidation is the most important function of sleep, either by challenging the evidence that consolidation is even a function of sleep or by claiming that other functions such as energy-saving, organ detoxification, wound healing and muscle tissue building are more important. Jerome M. Siegel’s hypothesis falls into the first type of disagreement. He hypothesizes that there is not enough evidence to support the claim that memory consolidation occurs during sleep. Above, we have two examples of evidence that proves memory consolidation occurs during sleep. The behavioural evidence shows that if sleep is prevented, memory consolidation is reduced. The neural evidence indicates that a sleep “replay” consolidates memory. However, Siegel argues against these evidences. An important scientific assumption is ceteris paribus that all other factors not investigated are controlled. The behavioural evidence does not control all other factors in the environment. For example, the act of disturbing sleep could have caused stress, which may have accounted for the reduced memory rather than due to only lack of sleep. Siegel also criticizes the idea that neural replay contributes to memory consolidation. He argues that if events when awake were to be consolidated, a reactivation of the identical mental experience should also occur. Fewer than 10% of dream reports referenced an experience learned. The experiences are rarely a “replay” of events, but instead the learning and emotions that were correlated to the experience. Hence, Siegel is doubtful on all memory consolidation evidence. Although it is reasonable for him to argue that external factors in these studies are not controlled, there is so much behavioural and neural evidence that his argument does not undermine the hypothesis completely.
From an evolutionary perspective, the reducing human’s responsiveness to stimuli that are potentially threatening during sleep is a danger to survival. This may lead us to believe that memory consolidation is not the most important function of sleep, as it is unlikely we could have evolved with this massive risk to our survival, just so that our long-term memories could be consolidated. Almost all animals sleep, and many argue the most important function of sleep is to increase the overall fitness of each organism. Apart from brain functions, some of these functions include energy-saving functions, thermoregulation, cell tissue repair and immune functions (Rasch and Born). REM appears to be largely associated to brain repair whereas NREM is for body restoration, including cell tissue repair and energy-saving functions. A study showed that sleep-deprived rats exhibited fewer white blood cells - the body’s main defense against sickness. In sleep-deprived humans, there are less than half of the protective antibodies after vaccination. This is evident from a study that showed that people with 8 hours of sleep per night were 3 times less likely to become ill than those with less than 7 hours of sleep. On top of making more white blood cells to attack bacteria and viruses, the brain triggers the release of hormones to encourage tissue growth. Metabolic activity during sleep is mainly anabolic (new molecules are constructed) instead of catabolic (molecules are broken down), which supports the idea that tissue repair occurs during sleep. Slow-wave sleep is associated with higher levels of growth hormones in the body, which are crucial for tissue repair and regeneration. The rate of healing, protein synthesis and cell division are greatest during sleep. Sleep promotes the optimal microenvironment for stem cells to develop. Physiological changes that occur during sleep include temperature, pH, carbon dioxide level and hormone production, which all assist stem cell homeostasis. The optimal physiological state makes an environment that promotes stem cell growth (Elkhenany et al.). Overall, sleep provides an ideal environment for cells to grow, maintaining overall health. A second important function of sleep is energy conservation. Sleep provides a period of downtime when energy can be stored and built up. People believe the requirement for sleep in mammals is linked to endothermy - the fact that they are warm blooded. Mammals need to conserve energy to maintain a high body temperature and active brain. During slow-wave sleep, the core temperature is reduced by one degree Celsius, so less energy is required to maintain our body temperature. This hypothesis is proven because metabolic rates are higher in smaller animals that sleep for longer hours.
Hence, it can be deduced that sleep serves multiple functions simultaneously. Memory consolidation is an important function, but a comprehensive understanding of the other functions must be taken into account. There are a lot of converging evidence that memory consolidation is a very important function of sleep, and it is appropriate for Siegel to conclude that external factors were not controlled in experiments, the variety and large amount of studies available allow us to conclude that memory consolidation is an important function of sleep. However, we have also seen sleep serving other important functions that are important to survival such as cell regeneration and energy conservation. Overall, we can conclude that memory consolidation occurs doing sleep, but it is not the most important function of sleep.
All things considered, despite knowing the importance of sleep, we know little about whether sleep is most crucial for memory consolidation. Some say consolidating memory is the most important function of sleep, whereas others dispute this and say memory is not a function of sleep, or is not the most important function of sleep. Both sides of the argument have experimental data to substantiate their claims. Taking everything into account, it can be inferred that sleep serves multiple functions simultaneously. Memory is one of these functions. Although memory is crucial for survival, it is not necessarily the most important function of sleep. We do not know if memory is the reason why we sleep, or if instead our sleep evolved for other reasons, and memory consolidation appeared later in evolution as a side effect. More research has to be done on humans as most current studies are backed up by rats’ behavioral and neural activities, which differ in some ways from humans. To fully comprehend to what extent sleep consolidates memory, the distinctions between the role of sleep in different species and how the roles interact needs to be better understood.
