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The Neurobiological Explanation of Sleep
Sleep accounts for about ⅓ of the human lifespan. It takes up a substantial part of our lives, yet most of us don’t know the answer to a common question; what happens when we sleep and why do we need to do it in the first place? Over the past decade, technological advances in molecular biology have allowed us to create a much more comprehensive picture of the genetic events and cellular mechanisms that underlie sleep's neurobiology. This article will discuss the critical role sleep plays in your general wellbeing as well as the science behind it.
The Importance of Sleep
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Before diving into any elaborate concepts or explanations, it is imperative to know and understand the answer to a simple question; why do we even need sleep? Sleep's importance can be attributed to multiple factors, but they mostly fall under the same general answer: rest has a profound impact on brain function. A sufficient amount of sleep is essential to cultivate brain plasticity—the brain's ability to adapt to input. Sleep deprivation enables our brain to function less efficiently during the day, thereby hindering our ability to retain information. Additionally, sleep facilitates removing waste products from the brain cells, leading to improved concentration, clear thinking, and memory accretion. There is also evidence that sleep can improve both short and long-term cognitive performance—promoting sharper thinking, and minimizing the probability of age-related cognitive decline. That being said, sleep doesn't only affect the brain. Studies have also shown that sleep deprivation can increase depression, seizures, high blood pressure, migraines, and can compromise immunity, which all pose potentially dangerous health threats to the body. To put it simply, sleep is essential because it allows your brain and your body to recharge, allowing you to feel refreshed and alert after waking up, reducing the risk of cognitive defects and fending off health risks.
The Four Stages of Sleep
Now that we understand why sleep is so essential, it is time to tackle the second question; what happens when we sleep? After falling asleep, our bodies proceed through a sleep cycle that includes four different stages. The first three sleep stages are generally known as non-rapid eye movement (NREM) sleep, while rapid-eye-movement (REM) sleep is traditionally characterized as the final stage.
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Stage 1 NREM (non-rapid eye movement)
When you first fall asleep, you enter stage 1 of NREM sleep. In this transitional stage, our body shifts from wakefulness to rest, with light sleep occurring accordingly. This is characterized by the cessation of muscle movement and the slow movement of the eyes behind the eyelids. Your muscles start to relax, and your heart rate, breathing, and brainwaves begin to decelerate. This is known as the "twilight" stage of sleep, where you are still subconsciously aware of your surroundings and can be awoken with slight disturbances.
Stage 2 NREM
The second stage lasts the longest, in which your body temperature decreases, your heart rate and breathing continue to slow down, and your muscles get more relaxed. This stage is when you become completely unaware of your surroundings and cease all eye movements as you fall into a deep slumber.
Stage 3 NREM
This stage is essential for rejuvenation and replenishment of the body and minds the next day. Bodily functions such as breathing, heartbeat, and brain wave activity are reduced to the lowest levels achievable, while muscles are relaxed to the utmost extent. This stage will be longer at first but will decrease throughout the night. Researchers believe that tissue reparation occurs during this stage and that hormones are released to promote growth.
Stage 4 REM (rapid-eye-movement)
Under rapid-eye-movement, your eyes will move back and forth very quickly underneath your eyelids, so your breathing rate, heart rate, and blood pressure will begin to increase. During this stage, dreaming occurs, and your limbs will become paralyzed - this is to prevent you from physically acting out in your dreams. Various studies have also connected REM sleep to memory consolidation - this process implicates REM sleep to transform recently acquired experiences into long-term memories. As you age, the REM stage duration will decrease, causing you to spend more time in the NREM stages. This stage aims to invigorate individual sections of the brain that are fundamental for memory and learning and act as a way for the brain to store, retrieve, and organize information.
Processes that Regulate Sleep
Sleep is regulated by two main processes; circadian rhythms and sleep drive.
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Circadian rhythms are 24-hour cycles that influence the body's internal clock, enabling essential functions and processes in the background. Among the most necessary and well-known circadian rhythms is the sleep-wake cycle. All body systems follow a constant internal clock (circadian rhythm) which is synchronized with a master clock in the brain. This master clock corresponds to an individual's daily cycles. It is directly influenced by external environmental cues, incredibly light, which is why circadian rhythms are linked to the process of day and night. The internal clock's primary function responds to these light cues on an individual basis, augmenting melatonin production at night and switching it off when light is detected. Circadian rhythms can promote consistent, restorative sleep when aligned correctly, but severe sleep disorders such as insomnia can arise when compromised.
Sleep drive is an equally important regulator of sleep because just as humans crave food, our bodies crave sleep. As you go about your day, your desire for sleep builds, and eventually, it must be satisfied. The critical difference between sleep and hunger: your body cannot interfere with your activities when you're hungry, but when you're tired, it can put you to sleep, no matter the situation. That is why getting an adequate amount of sleep every night (at least 7 hours, on average) is imperative to maintain a healthy and balanced lifestyle.
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Hopkins Medicine. (n.d.). The Science of Sleep: Understanding What Happens When You Sleep. Retrieved from https://www.hopkinsmedicine.org/health/wellness-and-prevention/the-science-of-sleep-understanding-what-happens-when-you-sleep#:~:text=The%20first%20part%20of%20the,fourth%20stages%20are%20deep%20sleep
Suni, E. (2020, September 25). Circadian Rhythm. Retrieved from https://www.sleepfoundation.org/circadian-rhythm
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Article author: Risheena Banerji
Article editors: Sherilyn Wen, Maria Giroux