General Neuroscience

Sleep - From Start to Finish: An Overview

Aditi Kona


Abstract

Sleep is a complex and dynamic process that is vital for the functioning of any human. On average, humans spend about one third of their lives sleeping, but the purpose of sleep remains largely unknown [1]. In recent years, research has brought to light many of the previously unknown mysteries of sleep. To this day, discoveries continue to be made as we try to uncover the details and inner workings of sleep. This article delves into the roles that varying structures play as well as the harmful consequences associated with the disruption of sleep. Furthermore, it discusses the factors linked with proper sleep and how one’s body responds through an average night.

 

Role of Genes and Neuro-transmitters in Sleep

Sleep is not a process controlled by a single structure or area of the brain. Instead, numerous parts work together in unison to achieve sleep. To start off, the hypothalamus, a peanut-sized structure deep inside the brain, contains groups of nerve cells that act as control centers affecting sleep and arousal. Within the hypothalamus is the suprachiasmatic nucleus (SCN) – a cluster of thousands of cells that receive information about light exposure directly from the eyes while controlling behavioral rhythm [2]. Damage to the SCN is linked with irregular sleep and is associated with feelings of sleepiness during the brightest times. This is due to the fact that the SCN is  not able to match the circadian rhythm with the light-dark cycle. Although it might seem that this same disconnect would be apparent in blind people, studies have shown that blind people retain some ability to sense light and are able to modify their sleep/wake cycle accordingly [3]. 

Another significant structure to discuss is the brainstem which is located at the base of the brain. The brainstem includes separate parts called the pons, medulla, and midbrain, each with its own function. Through communication with the thalamus, the brainstem controls the passage between consciousness and sleep. Sleep-promoting cells within the hypothalamus and the brainstem produce an inhibitory neurotransmitter called GABA (gamma-aminobutyric acid), which acts to reduce the activity of arousal centers in the hypothalamus and the brain stem. The brain stem (especially the pons and medulla) also plays a special role in REM sleep. It sends signals to relax muscles essential for body posture and limb movements so that we don’t act out our dreams. The thalamus behaves as a relayer for information from the senses to the cerebral cortex [4]. The distinction is that the thalamus stays silent during sleep, thus, allowing for peaceful rest without outside noise. In contrast, during REM sleep, the thalamus is actively sending the cortex pictures, smells, sounds, etc. to fill our dreams. There is much debate around whether the senses we experience through the day influence our dreams and continue to be an area of study. Regardless, the thalamus is responsible for providing the five senses associated with dreams. 

Next, the pineal gland, wedged within the brain’s two hemispheres, receives signals from the SCN and produces melatonin, a sleep-inducing chemical. People who have lost their sight and cannot coordinate their natural wake-sleep cycle using natural light can stabilize their sleep patterns by taking small amounts of melatonin at the same time each day. 

While mentioning the natural cycle of sleep, it is important to acknowledge the basal forebrain. This structure, part of the midbrain, works to promote sleep and wakefulness and acts as an arousal system. Release of adenosine (a chemical by-product of cellular energy consumption) from cells in the basal forebrain, and probably other regions as well, supports sleep drive [5]. Often, to keep themselves awake, people consume caffeine in the form of coffee or energy drinks. Caffeine counteracts sleepiness by blocking the actions of adenosine. 

Lastly, we discuss the amygdala, an almond-shaped structure that becomes increasingly active during REM sleep. The amygdala is often known as the section mainly responsible for emotions and feelings; however, its specific function in REM sleep has yet to be discovered.

 

Sleep Stages

As it turns out, sleep is an essential neurological requirement for our bodies. There are multiple neurological structures intricately involved in the triggering and control of sleep periods. There are two basic stages of sleep: rapid eye movement (REM) sleep and non-REM sleep (which is further divided into three stages). Each stage is characterized by specific brain waves, neuronal activity, and patterns of breathing and heartbeats. A single cycle goes through all stages of non-REM and REM sleep several times during a typical night, with increasingly longer, deeper REM periods occurring towards morning [6]. 

To start off, Stage 1 non-REM sleep is the changeover from wakefulness to sleep. During this several minute period, the body experiences relatively light sleep as heartbeat, breathing, and eye movements slow, and muscles relax with occasional twitches. At this moment, brain waves begin to slow from their rapid steady daytime beat. Stage 2 non-REM sleep is once again a period of change as the body transitions and enters a deep sleep. Both heartbeat and breathing slow, and muscles relax even further. Additionally, body temperature drops and eye movements stop. Since this is the longest stage, brain wave activity slows but is marked by brief bursts of electrical activity. Stage 3 non-REM sleep is arguably the most impactful stage as this is the period of deep sleep that allows one to feel reinvigorated in the morning. A person’s average heartbeat and breathing slow to their lowest levels during this period and muscles relax to the point where it is difficult to wake up. It is vital that the muscles are unclenched as it promotes an eased state of the body and allows for proper functioning. At this point, the body finally transforms into REM sleep which first occurs about 90 minutes after falling asleep [8]. 

REM derives its name from the vigorous side to side movement of the eyes behind closed eyelids [9]. The irregular frequency of brain wave activity resembles neuronal activity seen in wakefulness. Consequently, breathing becomes faster and irregular, and heart rate and blood pressure increase to near waking levels. However, the most notable characteristic of this stage is the vivid dreaming that occurs. Arm and leg muscles become temporarily paralyzed to stop one from acting out their dreams. Specifically, this is the stage that most people visualize when thinking of sleep; however, it continues to decrease in time as people age.

 

Sleep Mechanisms

One of the biggest questions is how the body regulates consciousness and sleep. How does it know when to transition into different stages of sleep or when to wake up? The answer to this lies in two key internal biological mechanisms: circadian rhythm and homeostasis. From body temperature and metabolism to the release of hormones, circadian rhythms control daily fluctuations. When someone naturally wakes up without an alarm, their circadian rhythm/ biological clock is at work. Overall, it controls the general timings of sleep and consequently leads to sleepiness at night time. Circadian rhythms synchronize with environmental cues (light, temperature) about the actual time of day, but they continue even in the absence of cues. The actual need for sleep is kept track of by a process termed sleep-wake homeostasis. The longer someone is awake, the stronger the sleep drive gets. Thus, sleep is longer and deeper after a period of sleep deprivation. Factors that influence sleep-wake needs include medical conditions, medications, stress, sleep environment, and what someone digests [10]. Perhaps the greatest influence is the exposure to light. In fact, specialized cells in the retina take in light while simultaneously sending signals to either advance or delay the sleep-wake cycle.  

Until about two decades back, sleep was just seen as resting that helped the body to recuperate. But the research studies about the brain and its functioning have led to some surprising ideas and conclusions about the biological necessity of sleep. So what and how does sleep help with the functioning of the brain? Specifically, what are the consequences associated with the disturbance of sleep?

 

Sleep Disruptions

Before we delve into the question above, it is vital to point out the damaging effects that a “clogged” brain can have. When the neurons in the brain are active during the day, they are consuming energy and produce by-products, like any living cell. Specifically, a toxic protein called beta-amyloid in brain tissue causes a variety of issues from slow functioning to deadly diseases. In fact, the accumulation of beta-amyloid is a characteristic common in Alzheimer’s patients. The body’s natural solution to this is sleep. Sleep activates a system that drains waste products from the brain. In turn, cerebrospinal fluid flows through the vessels surrounding blood vessels and removes beta-amyloid. This brain cleaning system was named glymphatic system as it was controlled by the shrinking and swelling of glial cells [11]. This indicates that the sleep activated cleaning system plays a role in the long term health and functioning of the brain. 

Prolonged disruptions of sleep also have other health related impacts on the body, both short term and long term - leading to daytime fatigue, stress, type 2 diabetes and increased risk of high blood pressure and heart conditions [12]. Sleep disruptions can occur from environmental factors like noise and lack of suitable accommodations for sleeping, besides innate causes like sleep apnea.


References


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  7. Sleep and Sleep Stages. Lumen Learning. https://courses.lumenlearning.com/wmopen-psychology/chapter/outcome-sleep-and-dreams/. Retrieved: 20/09/2019

  8. Marcin, Ashley et al. (26/07/2018). What is Deep Sleep and Why is it Important?. Healthline. https://www.healthline.com/health/deep-sleep. Retrieved: 20/09/2019

  9. Allik, J. Rauk et al. (2019). Control and sense of eye movement behind closed eyelids. PubMed. https://www.ncbi.nlm.nih.gov/pubmed/725508. Retrieved: 20/09/2019

  10. Medic, Goran et al. (19/05/2017). Short- and long-term health consequences of sleep disruption. PubMed. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5449130/. Retrieved: 29/09/2019

  11. (28/10/2013). How Sleep Clears the Brain. National Institutes of Health. https://www.nih.gov/news-events/nih-research-matters/how-sleep-clears-brain. Retrieved: 27/09/2019

  12. Sleep Disorders and Sleep Deprivation: An Unmet Public Health Problem. https://www.ncbi.nlm.nih.gov/books/NBK19961/. Retrieved: 26/09/2019

Aditi Kona

Aditi Kona


Aditi Kona is currently a junior from Kentucky. She's had a love for neuroscience after participating in science competitions and was named the 2020 Brain Bee State Champion. In school, she enjoys being part of STEM clubs like Science Olympiad and Women in STEM as well as outside of school such as in Science Fair. In her free time, Aditi loves to play the piano, dance, and volunteer at Norton Hospital.