Sleep architecture: The stages and cycles of sleep

 

hypnogram_updated with just 3 nonREM stages, wakefulness and REM

It may surprise some people that sleep is not a singular process. 

Rather, it is characterized by a series of patterns of brain activity that are distinctively different from what we encounter during the day while awake. 

Sleep is not really as simple as shutting down the brain and body.  In fact, many important brain and body functions take place during sleep that help to keep us healthy and alive. This is why sleep deprivation is hazardous; we literally cannot maintain good health and well-being without sleep. Sleep is not optional, but required for us to function.

Sleep architecture is one way that scientists can meaningfully understand the sleeping process. By using sleep architecture to define what falls within the range of normal sleep, they can consistently measure and map abnormalities in sleep architecture to form the basis for the diagnosis of a sleep disorder.


What is sleep architecture? 

Your sleep architecture can be thought of as a multidimensional map of your sleeping process. It's made up of patterns called sleep cycles, and within each of these cycles, you should ideally experience most, if not all, sleep stages. 

hypnogram (shown above) is a way to visually define the frequency and duration of sleep cycles and to identify the different sleep stages experienced over the course of the night. 

Sleep specialists use this rendering of our sleep architecture along with other documentation of sleep-related vital signs and physiological data to make their diagnoses. These include:

When you go into the lab for an overnight sleep study, your "hookup" is done to collect all of this other important data as well as to confirm the presence of sleep cycles and stages.

Sleep cycles

Sleep cycles are patterns of sleep stages which repeat, or "cycle," over the course of the night. If you have healthy sleep, you can expect to enjoy four or five sleep cycles per night. Each of these cycles phases in and out based on a variety of factors, such as: 

The quality of the sleeping environment. If it's too light or too noisy in your sleeping space, you may not be able to maintain consistent patterns of sleep due to arousals caused by external disruptions. Good sleep hygiene includes practices that help make your bedroom more conducive to quality sleep.

Any underlying health conditions you have. For instance, people with diabetes may experience imbalances in blood sugar and glucose which can easily disrupt sleep cycles.

The medications you are taking. You may be using multiple drugs or supplements which have an undesirable interaction; this can lead to frequent arousals or even the inability to fall asleep. Alcohol, cigarettes, marijuana, and opioid pain medications are also common substances that can interfere with sleep cycle patterns.

A normal pattern of sleep cycles (as shown in the hypnogram ) includes more stage 2 and stage 3 sleep at the beginning of the night and more REM sleep at the second half of the night. Brief awakenings are also considered normal.

Sleep stages

These are the actual kinds of sleep you achieve during each sleep cycle. There are officially two kinds of sleep in adults: nonREM sleep and rapid-eye movement sleep (REM).

It's also worth noting a third category that's carefully considered as part of the staging of sleep: wakefulness. Some wakefulness is expected over the course of a night of normal sleep, but large periods of arousal can indicate the presence of a sleep disorder.

In the nonREM category of sleep, there are three unique stages: 1, 2, and 3. REM sleep, as the name suggests, is a distinctive category of sleep characterized by apparent rapid-eye movements.

Let's first consider nonREM's three stages.

Stage 1 sleep

This is mostly what happens between all stages of sleep and wakefulness; it is usually shallow and transitional. During this stage, your brain begins to relax, but it still produces occasional bursts of alpha wave activity (which indicates wakefulness) between periods of light relaxation. 

Most of us experience Stage 1 sleep at a rate of up to five percent of our total sleep time. Stage 1 sleep is also known as N1 sleep.

Stage 2 sleep 

Normally considered the most common stage of sleep, Stage 2 sleep shifts toward waveforms which are signature markers of this stage of nonREM sleep. Our vitals (respiration, heart rate) tend to normalize in Stage 2 as we disengage from consciousness. Also, our core body temperature begins its normal decline as part of the circadian rhythm process.

We usually spend about half the night in this stage, also known as N2 sleep.

Stage 3 sleep

You may know this better as deep sleep. Doctors also call this slow-wave sleep (SWS) or delta sleep based on the unusual size and shape of the brainwaves that occur at this time. When our brains shift into Stage 3 sleep, we have begun to drop into full-fledged unconsciousness. Stage 3 signals the release of human growth hormone, which works at the cellular level to repair and heal the body and clean up waste products. 

Younger people enjoy more Stage 3 sleep because it is so vital to human development. As adults, we can experience this stage, also known as N3 sleep, for up to 25 percent of the night.  

 

What about REM?

Rapid-eye movement sleep (REM) is the time during our sleep cycle when we dream. REM is considered "active" sleep because the brain shifts into a highly activated pattern that produces varied and unusual waveforms distinctive to this stage. It's accompanied by muscular paralysis from the chin down; researchers theorize that this is to prevent us from acting out dreams.

If we are lucky, we spend about a quarter of our nights in REM sleep, which is critical to memory consolidation and learning. You can learn more about REM here.


As you can see, our sleep architecture tells a visual and data-rich narrative about our lives as we sleep. While there's a broad range of patterns and behaviors that identify normal sleep, it's when we can see the unusual variations in these patterns in the context of other physical data that we best establish the presence of nearly any kind of sleep disorder. 

 

Sources

American Academy of Sleep Medicine
National Institutes of Health
SLEEP

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