An introduction to sleep, including its stages and patterns.
Written by: Shriya
Is sleep actually so important?
Our doctors, parents, and teachers all tell us to get a good night’s sleep. We all know it is important, yet we continue to neglect the impact that a lack of sleep can have on our daily lives. We often find ourselves valuing finishing the homework assignment that's due the next day or watching a final episode of our favorite show over getting enough sleep. Perhaps this is because we haven’t completely comprehended the importance of sleep. So how critical is sleep actually? According to the National Institute of Neurological Disorders and Stroke, “without sleep you can’t maintain pathways in your brain that allow for learning, creating memories, concentration, and response.” In fact, sleep affects almost every type of tissue and system in the body – from the brain, heart, and lungs to metabolism, immune function, mood, and disease resistance. A chronic lack of sleep can therefore lead to various issues by increasing the risk of disorders including high blood pressure, cardiovascular disease, diabetes, depression, and obesity. These various impacts show just how essential sleep can be to a healthy lifestyle.
How does sleep work?
So we know sleep is important, but how exactly does it work? There are actually two types of sleep: rapid eye movement (REM) and non-REM sleep (which has three different stages).
Stage 1: This stage of non-REM sleep involves the transition from wakefulness to sleep. The period is short, lasting only several minutes. During this span of light sleep, your heartbeat, breathing, and eye movements slow, and your muscles relax with occasional twitches. Your brain waves begin to slow from their patterns throughout the day.
Stage 2: This stage of non-REM sleep involves a period of light sleep, which you experience right before entering into a deeper sleep. In fact, you spend more of your repeated sleep cycles in stage 2 sleep than in other sleep stages. Your heartbeat and breathing become slower, and muscles further relax. Your body temperature also drops and eye movements stop. Brain wave activity slows but is marked by brief bursts of electrical activity.
Stage 3: This stage of non-REM sleep involves the deep sleep that you need to feel refreshed in the morning. It occurs in longer periods during the first half of the night. Your heartbeat and breathing slow to their lowest levels during sleep. Your muscles are relaxed and you are difficult to wake. Brain waves become even slower.
REM sleep: This stage of sleep first occurs about 90 minutes after falling asleep. Your eyes move rapidly from side to side behind closed eyelids. Mixed frequency brain wave activity becomes closer to that seen in wakefulness. Your breathing becomes faster and irregular, and your heart rate and blood pressure increase to near waking levels. Most of your dreaming occurs during REM sleep, although some can also occur in non-REM sleep. Your arm and leg muscles become temporarily paralyzed, which prevents you from acting out your dreams. As you age, you sleep less of your time in REM sleep.
The first REM sleep episode in humans is relatively short. After the first REM sleep episode, the sleep cycle repeats itself with the appearance of non-REM sleep and then, about 90 minutes after the start of the first REM period, another REM sleep episode occurs. This rhythmic cycling persists throughout the night. The REM sleep cycle length is 90 minutes in humans, and the duration of each REM sleep episode after the first is approximately 30 minutes. (1)
What controls our sleep patterns?
Now that you know about the stages of sleep, you may be wondering how you wake up from the nightly cycle of sleep. The answer to that question are sleep mechanisms. Two of these mechanisms are sleep wake homeostasis and circadian rhythm.
Sleep wake homeostasis is a mechanism that keeps track of your need for sleep by reminding the body to sleep after a certain time. It also regulates sleep intensity which causes your sleep drive to get stronger every hour you are awake and causes you to sleep longer and more deeply after a period of sleep deprivation. Like most other things, sleep wake homeostasis is affected by a variety of things causing it to have a unique influence on every person. Some of these factors include medical conditions, medications, stress, sleep environment, what you eat and drink, and exposure to light. In fact, exposure to light is the most important. This is because specialized cells in the retinas of your eyes process light and tell the brain whether it is day or night. As a result, exposure to light can make it difficult to fall asleep and return to sleep when awakened.
Another mechanism that affects sleep is your circadian rhythm. You can think of circadian rhythm as your body’s biological clock, which controls your timing of sleep. It is what gives you the tendency to wake up in the morning at roughly the same time without an alarm. Your circadian rhythm directs a wide variety of functions from daily fluctuations in wakefulness to body temperature, metabolism, and the release of hormones. It synchronizes environmental cues such as light and temperatures with the actual time of day, allowing your body to link the day with sunlight and warmth and night with darkness and a lower temperature. Similarly to sleep wake homeostasis, your circadian rhythm can be disrupted by various things, including jet lag and even night shifts (2).
Throughout this series of blog posts, we will be further exploring the complexity of sleep to not only understand its value but also discover ways to improve it.
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