We all know the basic gist of sleep: what it is, what it feels like, and why it’s good. But most of us don’t dig much deeper than that baseline knowledge.
Which is exactly what we’re here to remedy!
In this second installment of our sleep series, we’re diving into the different phases of sleep. This is still a highly researched topic, and while many theories are largely based on circumstantial evidence, there’s a clear correlation between the quality of everyday functioning with the quantity and balance of these sleep phases.
Because one person’s sleep patterns can vary so greatly from the next, it’s difficult to focus on the specifics of an entire night’s sleep. (The duration and quality of each sleep cycle is heavily dependent on a variety of factors, including age, recent sleep patterns, or alcohol and drug use.)
Instead, let’s break down the average structure of sleep cycles to get a better understanding of what our bodies experience through everyday slumber.
The Four Stages of Sleep
To clarify, one full night of sleep actually consists of multiple sleep cycles — most people go through an average of 4-6 cycles per night. Sleep cycles are roughly estimated to last around 90 minutes, though the actual length of each cycle can be fairly variable.
Healthy human sleep consists of 4 clearly identifiable stages, all of which are broken into 2 main “categories”: non-rapid eye movement (NREM) and rapid eye movement (REM).
The first category of NREM sleep is technically comprised of 3 different stages, though each comes with their own unique characteristics and benefits.
Let’s get into it, shall we?
Stage 1: Light Sleep (Non-Rapid Eye Movement 1)
This first stage is what most people refer to as initial sleep onset or “light sleep,” though it’s more technically referred to as Non-Rapid Eye Movement 1, or NREM 1.
At this point, you’re the most susceptible to being roused from sleep; your senses are still relatively alert and vigilant to external stimuli. (If you’ve ever been interrupted when trying to fall asleep and said you weren’t actually asleep yet, that’s NREM 1.) This phase typically only lasts about 5 minutes in the initial sleep cycle to help slow down the activity in your brain and the twitching of your muscles.
The image here represents brain activity during NREM 1, with the highlighted portions identifying high amplitude theta waves (which, in simpler terms, are very slow brain waves). These brain waves indicate the brain’s effort to slow things down and bring you into a more relaxed state. As the night progresses and the person remains asleep longer, each NREM 1 stage generally decreases in duration with every sleep cycle.
Stage 2: Non-Rapid Eye Movement 2
Yes, the second stage of your sleep cycle is a continuation of the first (surprise, surprise!).
NREM 2 can last anywhere from 10-60 minutes; it’s most often characterized by a drop in body temperature, further relaxation in the muscles, and slowed breathing and heart rate. On average, about half of total sleep time usually consists of NREM 2 sleep.
As you can observe in the image above, this phase of sleep is noticeably different from NREM 1 in regards to brain activity. Identifiable brain wave patterns include sleep spindles and K-complexes, both of which are theorized to reflect significant moments of memory consolidation.
One study showed that individuals who learn new tasks tend to produce a significantly higher density of sleep spindles relative to those who don’t learn new tasks. (As we established with the previous installment of this series, higher quality sleep and consistency increases your memory consolidation and learning — and that’s most likely during this phase!)
Stage 3: Deep Sleep (Non-Rapid Eye Movement 3)
This is the phase associated with the deepest sleep, and it’s that sweet spot in your sleep cycle where you’re least likely to be woken up. Deep sleep is most likely to occur during the first half of the night as your muscles relax further and your pulse and breathing continue to slow.
NREM 3 is also identified by a distinct pattern known as delta waves, as pictured here:
These waves are noticeably more dramatic than previous wave patterns. Delta waves are indicative of a deep, dreamless sleep that leads to a lack of bodily awareness, making it an ideal time for your body to repair itself.
Researchers believe NREM 3 to be one of the most important sleep stages for body restoration; in fact, studies have shown that this particular stage of sleep is prioritized more immediately in individuals who had been deprived of sleep for a period of time. (Essentially allowing enough time for your body to catch up on its restoration.)
NREM 3 is often associated with increased secretion of human growth hormone (HGH), thus increasing tissue repair, tissue growth, cell regeneration, and circulation of immune cells. Each of these benefits require a significant amount of bodily energy, so they’re both most effective during restful sleep when your body is inactive and relaxed.
Stage 4: Rapid Eye Movement (REM)
The last identifiable sleep stage is known as REM sleep, taking up approximately 25% of sleep time in most adults. Most often, REM sleep intensifies the longer you sleep (i.e., you’re likely to spend more time engaged in REM sleep during the early morning hours before waking up).
Interestingly enough, average measures of brain wave activity during REM sleep can sometimes appear more active than brain waves recorded during waking hours:
Of course, that doesn’t mean you’re actually awake during REM sleep; it’s more that your brain is in a peak time frame for vivid dreaming, causing increases in brain activity.
You can think about it this way: if the NREM stages are optimal for restoring your body, the REM stage is ideal for mental restoration. There are plenty of unknowns related to REM sleep, but it’s currently believed to be essential for cognitive functions like memory, learning, creativity, and contributions to creative problem-solving.
One study assessed this theory by prompting two groups of participants to learn and retain a new task on varying levels of sleep deprivation. While both groups were taught the same association task, the group that was deliberately deprived of REM sleep showed little to no success in learning the task when prompted to perform it a second time. The group that had been deprived of NREM sleep, however, saw significant improvement between their first and second attempts at performing the task. These findings are indicative of just how imperative a role REM sleep plays in learning and association-related cognitive functions.
What is Sleep Architecture?
Alright — now that you know the main building blocks of an average sleep cycle, we can get to the structure of it all.
This is what’s known as sleep architecture: the fairly predictable organization and pattern of these 4 sleep stages throughout a full night’s rest. Generally speaking, NREM and REM repeat cyclically in healthy adult sleepers (so much so that many sleep disorders are actually diagnosed as a result of sleep architecture anomalies).
Take a look at this average sleep architecture timeline. As shown, each sleep cycle lasts approximately 90 minutes and ends with a variable period of REM sleep (marked by the yellow highlights).
While this image reflects a generally healthy sleep cycle in an adult, every individual will have a different looking sleep architecture, particularly in regards to duration and frequency. It’s even common to see variation within the same individual over time, so this is by no means the one and only “ideal” sleep cycle one should try to achieve. More than anything, these sleep cycle charts are helpful for catching bigger picture abnormalities, like skipping an entire sleep stage or experiencing too many sleep cycles per night.
(Also, let’s just take a minute to appreciate the double-meaning of “architecture” — not only is it the structural map of a sleep timeline, but it also visually looks like architectural structures. Kudos to the person who came up with such an ingenious name.)
Sleep Cycles and Athletic Training
Now you’re probably thinking: understanding the sleep cycle is all well and good, but how does that knowledge apply to improving training and performance?
In short, it depends.
We know, we know; that’s not the answer you were looking for. But with so much variability between each individual’s sleep cycles (and all the unknown factors that can influence them), there are no clear-cut recommendations on how to deliberately schedule sleep or nap times to improve your performance.
That doesn’t mean altering your circadian rhythm for better training outcomes isn’t impossible; it’s just important to take it with a grain of salt, since its effectiveness depends on the context.
For instance, athletes who have regular early morning training or competition may adjust their circadian rhythms to the “early bird” habit of going to sleep earlier to get enough shuteye. And, those who perform their best later in the evening may benefit more from a “night owl” sleep schedule (combined with additional factors like limited exposure to blue light to ensure they can still fall asleep at a reasonable hour).
Time to Catch Some Z’s!
Although sleep is an invaluable element to our everyday functioning, there’s still much to be researched when it comes to sleep cycles and sleep architecture. While smartwatches can currently approximate your sleep phases based on your movement and heart rate, it’s far from a perfected system.
If you find yourself pondering any sleep anomalies or concerns, it’s best to speak directly with a sleep specialist (just like how you’d speak to a physical therapist to address your movement!). They’ll be able to perform a more accurate sleep assessment and provide strategic steps to truly improve your overall sleep health.
And remember: sleep research is constantly evolving, and as wearable sensor technology also improves, there’s no doubt that sleep monitoring will become more reliable and accessible beyond the walls of sleep labs.
The more knowledge we develop, the more likely we are to succeed at adjusting our lifestyle habits to accommodate optimal sleep hygiene — which is exactly what we’ll cover in our next series installment! (So stay tuned!)