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A phenomenon that has left scientists baffled for centuries
Author: Eimaan Shahid
Every single day of our lives, many of us follow the same basic routine. We wake up, eat, do some work, enjoy some leisure time, eat again, and then fall asleep, only to be awakened 8 hours later to do the same thing over again. But during those 8 hours of necessary sleep, what really happens? Every night as we close our eyes, we feel ourselves slipping out of consciousness into a fictional world created in our minds known as dreams. The average lifespan of an adult living in the United States is 78.54 years old, and we spend about 26 of those years asleep (Campbell, 2017)! About a third of our lives is spent in this state of being neither alive nor dead, so what truly is sleep?
Sleep has been a difficult topic for scientists to tackle for centuries because of the fact that physically, a sleeping person seems inactive; any changes or activities to the body cannot be monitored from the outside. For centuries, this physical inactivity led researchers to believe that during sleep, the brain and body were completely dormant and the brain was inactive. Evidently, recent studies have proven otherwise. Although our bodies are resting, the brain is actually the most active when we are asleep. Scientists have concluded that “sleep is a state that is characterized by changes in brain wave activity, breathing, heart rate, body temperature, and other psychological functions” (Chivers, 2013).
To understand how the brain functions during sleep, it is important to grasp how our bodies get tired throughout the day. Our state of wakefulness is determined by two things: neurotransmitters and an organic compound known as adenosine (Sperlágh & Vizi, 2011). Throughout the day, neurotransmitters keep some parts of our brains awake by sending signals from nerve cells to target cells (Berry, 2019). When it is time to go asleep, the parts of the brain that are kept active appear to “shut off”. Adenosine is an organic compound that inhibits the function of these neurotransmitters. As the day progresses, adenosine levels slowly increase, causing us to get drowsier as the day goes on as the chemical slows the signals being sent within the brain by neurotransmitters. You can think of the adenosine as a lullaby, slowly putting our neurotransmitters to sleep as the day continues. Once we fall asleep, adenosine levels lower and the same cycle repeats day after day. It is widely known that substances like coffee, tea, and cola all help you stay awake for longer. They do this by containing a chemical called xanthine, which inhibits the effects of the adenosine and keeps neurotransmitters signaling normally.
Sleep can be split into two main categories: non-rapid eye movement (NREM) sleep, and rapid eye movement (REM) sleep. The main difference between these two kinds of sleep is brain waves and neural activity (“Brain Basics”, 2019). Let’s start with NREM sleep; this kind of sleep can be broken down into 3 subcategories. Stage 1 of NREM sleep occurs when you are transitioning between the state of being awake and falling asleep. When you first lie down in bed and your eyes are closed, but you can still easily be woken up, you are in this first stage of NREM sleep. The second stage, known as “light sleep”, is characterized by lower body temperature, and slower heart rate (Cherry, 2019). This light sleep stage typically lasts for 20 minutes at a time. The last stage is called “slow-wave sleep”, more commonly known to us as deep sleep. During this stage, body temperature, breathing, and heart rates drop even further as our body becomes temporarily immobile. Sleepwalking is also the most common in this slow-wave sleep state. The next kind of sleep, REM sleep. This is the most active period of our sleep marked by intense brain activity. During REM sleep, our eyes move rapidly and dart around. This stage is also where all of our dreams occur.
Researchers and scientists have spent centuries collecting evidence about what happens when we go to sleep, but the aspect of sleep that remains a mystery is why humans, and all animals in fact, need to sleep. Every night for 8 hours on average, we are in a vulnerable and completely unconscious state, so evidently taking our brain offline is worth the risk of putting us in danger. Many of the world’s profound geniuses like Albert Einstein, Da Vinci, and Thomas Edison, thought that sleeping was a complete waste of time, and all of them only got in 3 to 4 hours of shut-eye every night (“Genius Sleep Patterns”, 2020). Although sleeping does take up a third of our day, it is vital for the restoration of nutrients, energization, and restoration of memory in the brain. New York neuroscientist, Dr. Micheal Halassa, states, "It's sort of embarrassing that we have not figured it out yet. It's obvious why we need to eat, for example, and reproduce … but it's not clear why we need to sleep at all,” (Pappas, 2017). Researchers have developed a few theories about why we need sleep, the leading hypothesis dealing with plasticity; our ability to memorize, and learn. Recent studies that are working to decode the reason for sleep all revolve around cells in the brain called glial cells, which translates to “glue” cells (Pappas, 2017). Researchers have recently discovered that glial cells may have something to do with the flow of cerebrospinal fluid in the brain, and could be filtering out this fluid during sleep. These cells also outnumber neurons in the brain by three to one!
Using EEG tests, and MRI scans, researchers have gathered a lot of information about neurotransmitters, brain waves, and sleep stages to establish what sleep really is. Although we do not have a clear cut answer as to why humans sleep, it is evident that sleep is crucial to brain development, plasticity, and regulation of overall body functions. The next time you want to spend a little bit longer scrolling through Instagram before you go to bed, remember that sleep is our body’s natural process of recovery, and even though it’s purpose remains a mystery, sleep is the only way to rejuvenate and heal our bodies after a long day's work.
References
Berry, J. (2019, October 11). Neurotransmitters: What they are, functions, and psychology. Retrieved July 04, 2020, from https://www.medicalnewstoday.com/articles/326649
Campbell, L. (2017, October 19). We've Broken Down Your Entire Life Into Years Spent Doing Tasks. Retrieved July 04, 2020, from https://www.huffingtonpost.com.au/2017/10/18/weve-broken-down-your-entire-life-into-years-spent-doing-tasks_a_23248153/
The Characteristics of Sleep. (n.d.). Retrieved July 04, 2020, from http://healthysleep.med.harvard.edu/healthy/science/what/characteristics
Cherry, K. (2019, October 31). The 4 Stages of Sleep (NREM and REM Sleep Cycles). Retrieved July 04, 2020, from https://www.verywellhealth.com/the-four-stages-of-sleep-2795920
Chivers, T. (2013, December 13). How much do we really know about sleep? Retrieved July 04, 2020, from https://www.telegraph.co.uk/news/science/10494965/How-much-do-we-really-know-about-sleep.html
Pappas, S. (2017, July 18). Why Do We Sleep? Retrieved July 04, 2020, from https://www.livescience.com/32469-why-do-we-sleep.html
Sleep Advisor (2020, June 18). Sleep Habits Of The Greatest Geniuses Revealed (Einstein, Churchill). Retrieved July 04, 2020, from https://www.sleepadvisor.org/how-geniuses-sleep/
Sperlágh, B., & Vizi, E. (2011, April 11). The role of extracellular adenosine in chemical neurotransmission in the hippocampus and Basal Ganglia: Pharmacological and clinical aspects. Retrieved July 04, 2020, from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3179034/