Stages Of Sleep
We have an odd relationship with sleep. For it to be something that we do for 1/3 of our lives, we really don’t know much about it. So let’s talk about what makes up sleep.
What Are The Stages of Sleep?
To start, we need to establish that there are different levels of sleep. At any given moment our brains may be doing something quite different from the next. As a Sleep Professional, I’ve seen these stages thousands of times. We can say that our brains cycle between two forms of sleep, Rapid eye-movement sleep, known as REM, and Non-Rapid Eye-Movement Sleep, which is shortened to NREM. These very creative names mark the first distinction of sleep.
When a healthy person enters REM sleep, their body is paralyzed, as dreams and nightmares are more likely to occur in this stage. I’m sure you can imagine how much trouble there’d be if we all acted out our dreams. Well, you’ll be frightened to know that’s a thing. It’s called REM behavior disorder. We’ll talk about that in detail another time.
As for now, We can further divide these 2 forms of sleep into 4 total stages. N1, N2, and N3. The N signifies that these 3 stages are a subset of NREM. The fourth stage is just REM.
These stages are all differentiated by different brain waves and different physiological reactions in the body. We used to have 5 stages of sleep, with there being a fourth stage of NREM, N4, but over time it seemed that N4 wasn’t drastically different from N3, so these labels were merged.
Our bodies cycle through the two forms of sleep, normally starting with NREM. A complete sleep cycle of NREM and REM can last for about 90 to 110 minutes. NREM typically makes up the majority of the cycle in the early parts of the night. Though we don’t always go through the stages in order, generally we always pass through NREM first. Going straight into REM at sleep onset can be a sign of sleep deprivation.
Falling asleep too quickly is another sign of sleep deprivation. In general, it may take a healthy adult anywhere from 10-20 minutes to fall asleep. Anything faster than this may be a sign that you aren’t getting enough sleep, or that the quality of your sleep is lacking. To some, this may sound odd. Many people have a goal to fall asleep as quickly as possible, and they feel like something is wrong if this doesn’t happen. I plan on discussing our society's misunderstanding of sleep latency in the future. For now, let’s get back to the sleep stages.
What Does Each Stage Do?
After the initial sleep latency, most people will go into N1. Here the brain has replaced the alpha waves that dominate relaxed wakefulness with theta waves. It’s worth noting that when we are awake with our eyes open, beta waves are our dominant “pattern”. When we are awake with our eyes closed, alpha waves begin to appear. So this transition from alpha to theta waves is an important marker that we are falling asleep. People don’t usually stay in N1 for very long, it seems to take up about 3-8% of our total sleep time. From what I’ve seen, N1 rarely lasts for more than two minutes. This stage is fragile, so it’s easy to wake someone up while they’re in N1.
Next is N2, which makes up about 45-55% of our sleep. N2 is characterized by Sleep Spindles and K-complexes along with a lower heart rate & body temperature. K-complexes and Sleep Spindles are electrical signals that appear in the frontal and the central regions of the brain, respectively.
You may know of the frontal lobe, by the job it performs while we’re awake. The frontal lobe is generally where functions like emotion regulation, planning, reasoning, and problem-solving occur. The frontal lobe is also where K-complexes are produced. K-complexes are tall delta waves that last for about one second. They seem to act as a way to maintain sleep. It’s common to see them appear after a person has been slightly disturbed, almost as if they’re saying “Shhh, go back to sleep.” There is also some research showing that they play a role in memory consolidation.
Sleep spindles are brief bursts of electrical activity that originate from a mix of the temporal lobe and frontal lobe, normally called the central region in the sleep field. Sleep Spindles are believed to play an important role in synaptic plasticity. Synaptic plasticity is vital to our brain's ability to learn and adapt to new things. Studies also seem to show that sleep spindles play an important role in memory consolidation, just like the K-complex.
So it seems that going into N2 is important to our ability to learn, adapt, and remember new things.
N3 is primarily identified by the heavy presence of slow, tall, delta waves. This has earned it the nickname, slow wave sleep. These waves originate in the frontal lobe, but, mainly in children, they can be so dominating that our sensors dedicated to other parts of the brain also start to pick them up.
N3 is considered the deepest stage of sleep, and it's the hardest to wake up from. Research has shown that this is the stage when the body repairs and regrows tissues, builds bone, and muscle, and strengthens the immune system.
So if you're exercising regularly, and you should be, this stage is vital to your recovery.
This stage has so many benefits for the body, so it's a little depressing to know that as people age, they tend to spend less time in this slow wave sleep and more time in stage N2 sleep. It makes up 15-20% of our sleep.
I'd like to look into it further. At the moment, I don’t believe aging is the only factor that decreases our slow-wave sleep. I’ve had a wide variety of patients over the years. And I’ve seen some 50 y/o’s with no SWS worth noting. I’ve also seen some 70 y/o’s that have quite a bit of SWS. It feels like other factors are in play other than just aging. At the moment, I think long-term alcohol intake may also have an effect.
I digress, If you do manage to wake someone up out of N3, you can expect for them to have some sleep inertia. More than likely they’ll be a little confused, and slow on the draw for a few minutes after waking up.
This is also the stage when sleepwalking, night terrors, and bedwetting occur. Sleep Apnea, and possibly depression can also disrupt N3, causing a person to get less than they should.
Now We Can Get Into REM.
During REM, beta waves become the dominant waveform, causing it to look quite a bit like being awake, with eyes open. Though the brain waves are similar to wake, the skeletal muscles are stricken with atonia. Atonia is the term for reduced muscle tone in many of the body's muscles; This is a normal function of REM sleep, most likely to keep us from acting out our dreams in real life. Though our muscles are paralyzed, Our eyes remain quite active, moving…rapidly. You guess how REM sleep got its name. This stage usually starts 90 minutes after you fall asleep, but, again, sleep-deprived individuals go into REM much sooner. The first REM period typically lasts 10 minutes, with the final one lasting about an hour. In total REM makes up 20-25% of our sleep.
REM, like N3, is an important stage of sleep. Studies have shown that REM is important for memory consolidation, and some posit that it helps us to process our emotions.
What Affects Our Sleep Stages?
Many things hurt our sleep. Poor sleep hygiene, noise pollution, and stress all lower the quality of the rest that we get. But some things can affect specific stages of sleep.
REM and N3, are especially prone to disruption. This is concerning, since these two stages at first glance, seem to offer the most benefit. Disruption of N3 would impair our body’s ability to heal and take care of our tissue while messing with REM sleep would leave us without the ability to relieve our mental state. Therefore we should know what to be on the lookout for.
Alcohol intake before bed can lower the amount of REM sleep that we get. Alcohol wreaks havoc on our sleep in general, so this is just another reason to avoid drinking before bed. If you need extra reasons to avoid traumatic brain injuries, studies have shown that, along with other damage to our sleep cycle, people with TBI tend to get less REM sleep.
Studies show that depression can increase the amount of REM sleep we get, at what appears to be the cost of N3/SWS.
Overall, sleeping is more complicated than it may first appear. Even though we are resting and recovering, our brain is quite busy. As shown by our removal of stage N4, our understanding of sleep stages is ever-evolving. Though it seems that N3 and REM are the “most” important sleep stages, this may be a hasty assumption to make, as it's likely that N2, and even N1 provide undiscovered benefits.
SOURCES:
https://www.verywellhealth.com/the-four-stages-of-sleep-2795920
https://pubmed.ncbi.nlm.nih.gov/24395522/
https://www.ncbi.nlm.nih.gov/books/NBK19956/
https://link.springer.com/article/10.1007/s11910-013-0430-8
Physical copy of Polysomnography for the Sleep Technologist, Instrumentation, monitoring, and related procedures.
