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Sleep is vital for many aspects of our lives, and we spend around one-third of our time doing it. We have all felt the consequences of staying up late, and sacrificing sleep for something else. The next day, we would wake up moody, disoriented, regretting our decision to deprive ourselves of rest and looking forward to the next time we can snooze.
Besides the obvious benefits that sleep has on our health, scientists have been wondering if sleep affects our memories and if it does, is it necessary for remembering new information.
During the initial studies, when sleep was shown to have a positive impact on memory, scientists thought that it was a passive role. Spending time resting meant that you were cut out from the environment and that there were fewer sensations to interfere with your memory. Newer studies showed that sleep plays an active role in the consolidation of memories, and while earlier studies focused more on the REM sleep and its importance, further studies highlighted the importance of deep (slow-wave) sleep.
We have learned a lot about sleep and memory from years of studying, but there is still a lot of research to be done.
Types of Memory
To process external information, store it as a memory and then be able to retrieve it, is a fundamental ability of all living creatures. This potential is needed so that all living things could adapt to the changing environment. We can say that a better memory is an adaptive feature that was supported by natural selection, and we see this storage of information on every level. For instance, if your immune system cells weren’t able to recognize some things, fighting off bacteria and viruses would be a lot less effective.
When it comes to humans, not all memory is the same. There are different parts of our central nervous system involved in remembering other kinds of information. Generally, hippocampus, amygdala, and neocortex play a central role in memory storage.
Depending on the involvement of different parts of our brain, there are two types of distinguished memory; declarative and nondeclarative.
There are two types of declarative memory: episodic memory that is referred to storing information about events, and context-based memories; and semantic memory that enables us to remember random fact and things that are independent of contextual knowledge. Declarative memories can be encoded even without our intention, but are exclusively retrieved by active, aware attempts. Episodic memories can be learned quickly, in just one try, in contrast to semantic memories that need repeated encoding.
Unlike declarative memories, nondeclarative ones can be obtained without the involvement of medial temporal lobe structures, and they rely on the different parts of the brain. Nondeclarative memories include procedural memories for motor and perceptual skills and certain forms of conditioning and learning. These memories can be obtained and retrieved without awareness, but the learning process is slow, and it usually requires many repeated attempts.
Distinguishing between nondeclarative and declarative memory can be quite tricky, as brain parts involved in both of these processes are communicating all the time, and that interaction is necessary for learning.
How Are Memories Stored?
There are three processes connected to memory: acquisition, consolidation, and recall. While acquisition and recall happen during waking hours, consolidation is mostly associated with sleep. Consolidation means moving short-term and easily lost memory, to better preserved long-term memory. The way that memory is preserved or erased is by strengthening or weakening neural synapses.
Initial studies focused more on the effect that REM sleep had on memory. Later on, it became clear that all three stages: light, deep and REM sleep were important for constructing memories. While light and deep sleep play a significant role in the embedding of declarative memories, REM sleep is vital for nondeclarative memories. Some even argue that the cyclical changes of these sleep phases are what makes memory possible.
Sleep is a perfect time for storing memories, as we are disconnected from our surroundings, and don’t have new information coming at all times and interfering. We are bombarded with information at any given moment, and our brain has to decide what’s important and store that somehow. The suggested model of consolidating information is known as a two-stage memory system. Firstly, we have a fast learning store (hippocampus), where the encoding of memories is quick and efficient. There is no more than one trial needed, but this information can quickly get lost when there is a new amount of information coming. Our brains had to find a way to overcome that somehow and store the memory for good. That happens slowly, as the data passes to long-term memory in neocortex. During our sleep, new memories are repeatedly reactivated, and that’s how they slowly get embedded in long-term knowledge.
The time for memory to go to long-term storage can vary between a day, and several months or even years. That depends on the acquired information and preexisting neural networks in our brain.
Sleep Deprivation and Memory
Sleep is essential for all of the processes related to memory. When you are sleep deprived, your concentration and cognitive ability are affected. That’s why it is hard to acquire new information when you are tired. Consolidation of memories is also affected, as less sleep means less time for memories to go to long-term storage. This way, information will stay in the hippocampal synapses, and it’s more likely to become affected by the new sensations and get lost in the meantime. Fatigued individuals find it harder to recall already stored information, so sleep deprivation affects memories on every level.
It is still not clear about all of the mechanisms that sleep deprivation affects memory, but it is suspected that adenosine build-up has a lot to do with it. Caffeine is showed to fight off the effects of adenosine successfully, and that is probably why so many students rely on coffee and energy drinks during their study sessions.
It seems that lack of sleep weakens some neural circuits, and that’s why our memory is affected. But don’t worry, our brain has an ability to reconsolidate memories, so during your next resting period, it will work hard on repairing those synapses.
People who have insomnia often have memory problems. That is another reassurance that sleep is the key element in remembering things.
Sleep and Learning
Electroencephalogram (EEG) is used for measuring brain activity. It gives us a good insight into what happens in our brain during sleep. EEG readings of Stage 2 of non-REM sleep show short bursts that scientists have name Spindles. Each spindle lasts for about a second, and there could be thousands of them each night. Spindles are correlated with the transfer of memory from the hippocampus to the neocortex and formation of long-term memory.
The density and number of Spindles are correlated with intelligence, and the number is observed to increase during the process of learning. As we age, the number of Spindles during our sleep declines, which is thought to have a connection with the cognitive decline in older people.
Light stages of sleep (Stage 1 and 2) are essential for motor memory. They also help the brain remain plastic to getting the new information. The light phases of sleep are usually shorter during the first part of the night, and then they become longer as you approach the dusk, so cutting short on your sleep may not be a good idea.
Naps are an excellent method to boost your learning abilities. They are especially beneficial to our procedural memory.
The Link Between Memory, Dreams, and Cortisol
Dreams have been a mystery for us for a long time. No one is quite sure why they happen and what can be their purpose, but some scientists have argued that they are playing a role in consolidating our memories.
During the waking hours and most likely REM sleep, the information flow is from neocortex to hippocampus. However, during the slow-wave sleep, the information flow is mostly reversed, but there are still some data coming to hippocampus from neocortex.
Dreams mostly occur during REM sleep, but they are shown to happen during other stages as well. It is just that these dreams are entirely different. Dreams connected to non-REM sleep are short, often episodic and are mostly memory based. That is why scientist argue that slow-wave sleep is maybe the most critical stage for memory consolidation. The neural circuits repeatedly reactivate, and that way your brain is strengthening the memories, while you experience dreams.
Dreams during REM sleep are entirely different; they are more chaotic, fragmented and can be far away from your memories and reality. That’s why some bizarre dreams happen, where you can fly off a building, or blink through different places on Earth in a matter of seconds.
Scientists suggested that the high level of cortisol as long with some other neurotransmitters can alter our memory and dreams. Cortisol is a hormone produced by the adrenal glands, located on your kidneys. It is created in times of stress, and it has an alarming function on your body. High levels of cortisol are shown to disrupt memory consolidation and as well as affect dreams. It is interesting how patients with high cortisol levels similarly described their dreams to individuals who have experienced some traumatic event. These descriptions, are often fragmented, bizarre and non-coherent.
While a lot about memory and sleep is still a mystery, scientists are doing their best to understand this phenomenon better. There is a large body of research being done in this field at the moment. While we are waiting for the results, one thing is clear; we need sleep to store memories. Sleeping well will improve your mood, concentration and give you proper motivation to deal with new stuff daily. If you are well rested, you’ll find it easier to learn things, and your brain will have more time to make sure that you don’t forget them.
If you want best cognitive results positive, stick with a regular bedtime routine, remove distractions from your bedroom, relax before going to sleep, and don’t forget to eat healthily and exercise regularly. If you have time during the day, consider taking a nap, as they can also be beneficial to you.
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Dusan is a biologist, a science enthusiast and a huge nature lover. He loves to keep up to date with all the new research and write accurate science-based articles. When he’s not writing or reading, you can find him in the kitchen, trying out new delicious recipes; out in the wild, enjoying the nature or sleeping in his bed.