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How Does Sleep Work? Essential Info from Sleep Doctor

Every single one of us needs a good night’s rest. Yet unfortunately the basics of how sleep works often isn’t covered in health classes. Understanding sleep and its various facets is not just an academic pursuit but has real-world implications. The quality of our sleep directly impacts our alertness, energy levels, and mood the following day. Moreover, chronic sleep deprivation can lead to serious health complications. Thus, delving into the mechanisms of sleep can empower us to make informed decisions that enhance our sleep quality and overall life satisfaction. In this article let’s do a deep dive into what’s going on in the various sleep stages, what benefit we get from each, and how we cycle through all the sleep stages when we are getting healthy sleep. 

The Stages of Sleep

There are four stages of sleep that we cycle through approximately 4 to 5 times each night. These stages are categorized into Rapid Eye Movement (REM) or Non-REM (NREM) sleep. Each of these stages plays a unique role in the sleep cycle, contributing to the restorative functions that sleep provides.
How Do Sleep Studies Work
A sleep study is required to measure your sleep stages. This would be done in a sleep disorders institute. There are wearable devices now, though use caution in using the data as they are not 100% accurate. A sleep study will give the most accurate information.

Stage 1: NonREM 1

This first stage of sleep is the lightest stage and usually lasts for about 5-10 minutes. In a normal healthy sleeper, Stage 1 sleep makes up only 3-5% of their total sleep. During this stage, we are in a transitional state between being awake and asleep. Our brain produces alpha and theta waves, and our muscles begin to relax. It's easy to be awakened in this stage, and if we are, we might not even realize we had started to doze off.

In NonREM 1, we sometimes experience sudden muscle contractions, which can give a sensation of falling. They are called “hypnic jerks.” These twitches are normal and are often accompanied by a feeling of surprise or alarm. As we move past this initial phase, our body prepares to enter a more profound and restorative state of sleep.

In people with sleep disorders such as obstructive sleep apnea or periodic limb movement disorder, they will often have more Stage 1 sleep. This is part of why they will feel like slept “lightly” and are not refreshed the next morning.

Stage 2: NonREM 2

In Stage 2 sleep our brain activity starts to slow down, and our body temperature decreases. This stage typically comprises 45-45% of our total sleep time. During NonREM 2, our heart rate and breathing rate also slow down as we edge closer to deep sleep.

Sleep spindles, which are bursts of rapid brain activity, characterize this stage. These patterns are thought to be involved in consolidating memories and processing information for learning. Actually, a greater density of sleep spindles is associated with better learning. As we spend more time in this stage, we become less responsive to external stimuli, setting the stage for the deep sleep that follows.

People with sleep disorders may also have a higher amount of Stage 2 sleep than normal, as they have less Stage 3 sleep and less REM.

Stage 3: NonREM 3

NonREM Stage 3 is the deepest stage of sleep, and it is during this stage that our bodies do the most restoration and repair. Our brain produces slow delta waves, which are very high amplitude. Because of the brain waves, this stage of sleep is also called “slow wave sleep” abbreviated SWS, or “deep sleep.” This gives us a feeling of sleeping really deeply, and can make it hard to wake up. If you ever turn your alarm clock off and keep sleeping it’s likely you were awoken during deep sleep. Children have much more deep sleep than adults, and seniors have very little. We are the least responsive to stimuli such as sounds, or in the case of small children being moved, during deep sleep.

This stage is essential for feeling refreshed and energized during the day. The body's healing processes are most active. Many physiological processes are slowed down and very regular, including blood pressure, heart rate, respiratory rate and stress hormones. There are two other unique features of Stage 3 sleep that contribute to healing.

Restorative Value of NonREM

In the brain there is a lymph system, called the glymphatic system. During sleep it opens up to almost twice it’s size, and cleans out metabolic waste products from the brain. One of the compounds that’s cleaned out is beta-amyloid protein. This is significant because beta-amyloid proteins can build up into plaques, and these plaques are implicated in Alzheimer’s disease. We’ve known for a long time that there’s an association between poor or short sleep in earlier life with the development of Alzheimer’s disease later in life. It’s thought that the glymphatic system may not have enough time to clean the brain.

The other restorative method that’s most active during deep sleep is growth hormone. A total of 75-85% of our total growth hormone is secreted during deep sleep. Even for those of us who aren’t growing taller, we still need growth hormone to repair during sleep. Skin, bone, muscle and more are all effected by growth hormone. Along with the decline in deep sleep seen with aging is a decline in total growth hormone.

REM

Rapid Eye Movement or REM sleep is in a category of its own. REM sleep is also called paradoxical sleep because the brain is very active, yet the skeletal muscles are paralyzed. Brainwaves during REM are most similar to wake in that they are fast and low amplitude or short. This is why we can have really complex, story like dreams. Thankfully, our skeletal muscles are paralyzed so we can’t act out our dreams. There is one rare sleep disorder called REM Behavior Disorder in which the skeletal muscles aren’t paralyzed, and people do act out their dreams, which can be a danger both to themselves and others. (More on that in another article!)

In normal healthy sleepers about 20% of their total sleep is REM. REM provides a couple very important functions. First, REM is very important for learning and memory. When we learn something during the day, it’s during REM sleep that the new information is incorporated into our overall understanding of that topic. The other important role of REM is in emotional regulation. It helps us remember things, but also prunes the emotional charge of the memory. People who have PTSD (post-traumatic stress disorder) have altered REM, so the emotional aspect of the traumatic experience does not fade with time.

These functions are also why it’s particularly important for children to get the recommended amount of sleep. If they are sleep deprived, they will get less REM, which can translate into learning and behavior problems during the day.

Sleep Cycles

Approximately every 90-110 minutes we cycle through all of these sleep stages. The normal sleep cycle starts with a minute or two of Stage 1 sleep, then some Stage 2 sleep, followed by stage 3 sleep. Next we’ll go back up to Stage 2 sleep, then have some REM. Often at the end of a sleep cycle people will wake briefly. This awakening is so short that we’re not aware of it. During that brief awakening people will often open their eyes, look around briefly, maybe adjust the covers and go back to sleep.

In the first half of our sleep, the Stage 3 periods are longer, with shorter REM periods. In the second half of our sleep that pattern is reversed, with longer REM periods and shorter Stage 3. The body prioritizes that physical restoration of deep sleep by having more of it at the beginning of the night. When people are sleep deprived they will have less REM sleep.

This pattern is called our “sleep architecture” and which is graphed out on a “hypnogram.” There are changes to sleep cycles across the life span. Infants have a shorter sleep cycle of 70-90 minutes. There are also changes in how much time we have in each sleep stage, let’s look at that next.

Changes Across the Lifespan

Sleep changes across our life span, giving us a different sleep experience at different times of life. Here are typical changes:

NonREM Stage 1: Tends to increase in our senior years. Will also increase with any sleep disturbance, whether it be a sleep disorder, or disruptions from the sleep environment.

NonREM Stage 2: Stays stable across the lifespan in healthy sleepers. Often Stage 2 sleep will increase with a sleep disorder, as Stage 3 sleep and REM sleep decline.

NonREM Stage 3: This is where some of the biggest changes are seen. First, in adolescence Stage 3 will decline by 40% as a child moves through puberty. Then again in our senior years deep sleep declines again. Brainwave amplitude of Stage 3 sleep decreases with age, which may be part of why seniors often report not feeling like they are sleeping as deeply as they did earlier in life.
REM: REM stays fairly stable across the lifespan.

Changes in Sleep Disorders

Note that sleep disorders often change our sleep by reducing both Stage 3 and REM sleep. These reductions translate into some of the consequences of sleep disorders, both feeling poorly day-by-day, and the long-term health consequences of lost sleep.

Become a Skilled Sleeper

Struggling with sleep disturbances such as sleep apnea or sleep paralysis? Our comprehensive "Skilled Sleeper" course, led by Dr. Catherine Darley, offers expert strategies and insights to help you overcome sleep challenges and enhance your sleep quality. This course includes interactive sessions, personalized advice, and practical tools tailored to your specific sleep needs.

Enroll today to start your journey towards restful, uninterrupted sleep. Gain the knowledge and skills you need to master your sleep patterns and enjoy a healthier, more energized life.



About the Author: Dr. Catherine Darley

Dr. Catherine Darley is a pioneering internationally recognized expert in the use of natural, behavioral and lifestyle medicine for the treatment of sleep disorders. Dr. Darley founded the Institute of Naturopathic Sleep Medicine to fill the need for natural, less invasive solutions to a common problem–poor sleep.  

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