Sleep Apnea and Sleep Stages

By Anne – Registered Polysomnographic Technologist

When asked if they sleep well, most people will report on their sleep duration only. They will say, “Yes, I slept for seven hours last night” or “No, I only got three hours of sleep.” 

While the duration of sleep is definitely important, the quality of that sleep is even more so. Consumer wearable devices, such as a Fitbit or Apple watch, can give you an idea of whether you are in light sleep, deep sleep, REM sleep, or awake, but these are just educated guesses.

The only true way to know is by using an EEG (electroencephalogram) to monitor the waves of the brain during each sleep stage. As a result, this is not typically a viable metric to track nightly, so we settle for these estimates.

If you’ve heard of one sleep stage, it is probably REM sleep – Rapid Eye Movement. It is the most easily identifiable stage. People often talk about deep sleep, as well, and often confuse deep sleep with REM sleep. 

There are, in fact, 5 sleep stages. However, we typically only monitor 4 of the stages now that two were merged together recently. Those stages are divided into two categories: REM sleep and non-REM sleep.

Polysomnography (PSG for short) means “multiple sleep recordings” and is exactly that. During polysomnography, the technologist will record the following metrics:

• Your brain waves using EEG – electroencephalography
• Your heart rhythm using ECG – electrocardiography
• Your eye movements using EOG – electrooculography
• Your muscle tone and limb movements using EMG – electromyography
• Your chest and abdominal movement
• Your snoring, blood oxygen level, breathing pattern, and body position. 

All these measurements give the technologist a thorough look at the quality of your sleep as well as the cause of any disruptions. This allows the doctor to diagnose your sleep condition. Of these monitors, the ones used for monitoring the stages of sleep are the EEG, the EOG, and the EMG.

While not technically a “sleep” stage, wake time is very important to get the most accurate picture of a person’s sleep. During polysomnography, we can flag the time a patient is awake and remove it from consideration when looking for disordered sleep. 

It is also valuable to see how often a patient wakes up as well as try to narrow down the cause of waking. A patient may wake after a respiratory event (like an apnea) or after a limb movement or even just naturally wake up after a REM cycle. 

Noting these periods is very important also as typically this is the only stage that a patient will be able to speak. Patients will report having woken multiple times a night to go to the washroom or rollover.

When identifying this stage on a PSG, we look for a low voltage, mixed frequency activity in the brain waves accompanied by alpha waves (8-13Hz) generated primarily from the occipital lobe of the brain. 

We would also note high muscle tone on the chin muscle. Patients may or may not open their eyes or be conscious of this waking up, and it must last for 16 seconds or more to be counted as being a “wake stage.”

The first stage of sleep is non-REM sleep stage 1, or N1 sleep. This is a very brief stage of sleep that lasts usually for only a few minutes. The brain and body start to slow down and relax. You may still be aware of your surroundings and can very easily wake up during this stage. 

You may experience muscle contractions or jerks in this stage. It will usually be a very small part of the night and is usually classified under wake or light sleep on consumer wearable devices.

When identifying N1 sleep and sleep onset, the registered polysomnographic technologist would be watching for when the alpha waves of the wake stage subside and vertex sharp waves (sharply contoured waves with a duration of fewer than 0.5 seconds) begin to appear from the central lobe of the brain. Muscle tone will still be quite high during this phase.

The second stage of sleep is non-REM sleep stage 2, or N2 sleep. This sleep stage is where you will spend most of your night. Your respiration and heart rate will start to slow down, and your body will have a slight drop in temperature. All of this can be measured using your consumer wearable device – like a Fitbit – particularly the heart rate slowing. This is how they tend to note that you are in “light sleep.” This light sleep stage should be 40-60% of the night overall.

When identifying N2 sleep, the registered polysomnographic technologist would be watching for sleep spindles and K-complexes. The K-complexes look like well-delineated negative sharp waves immediately followed by a positive component with a duration greater than 0.5 seconds from the frontal lobe. 

Sleep spindles are a cluster of fast waves (11-16Hz; but usually 12-14Hz) with a duration greater than or equal to 0.5 seconds from the central lobe.

The third stage of sleep is non-REM sleep stage 3, or N3 sleep. This stage is considered to be a deep sleep. It is significantly harder to wake someone from this stage. Muscle tone is relaxed, pulse and breathing rates are lower. 

This stage is one of the most critical stages and typically will occur in longer stretches earlier in the night and in shorter periods later in the night. This is due to the body prioritizing this stage as it is when the body rebuilds and restores itself. 

This stage is also crucial for memory and brain restoration. Short-term memories are converted into long-term memories, and the pathways to access the information are strengthened. The amount of N3 sleep that a person will get over the course of the night will decrease as the sleeper ages. Many elderly people will have no N3 sleep at all.

When identifying N3 sleep, the registered polysomnographic technologist watches for large delta waves (0.5 to 2 Hz) with an amplitude greater than 75 µV originating in the frontal lobe of the brain. 

When these waves cover more than 20% of the epoch, it is classified as N3 sleep. When the waves cover more than 50% of the epoch, it was originally classified as N4 sleep; however, the American Association of Sleep Medicine has since merged these two stages together as N3. 

As we age, the amplitude of the delta waves decreases until it no longer meets the criteria for scoring and N3 sleep becomes absent.

The last stage of sleep is rapid eye movement sleep or REM sleep. REM sleep is a very active stage of sleep. Heart rate and respiration become irregular, blood flow to the brain increases, and the eyes shift back and forth quickly. Muscle tone relaxes to zero. 

This stage is when dreaming occurs. Adults will typically spend about 20% of their night in the REM stage of sleep. REM sleep benefits memory, learning, and mood stability. A lack of REM sleep can increase migraine occurrence and obesity.

When identifying REM sleep, the registered polysomnographic technologist would watch for the back-and-forth movement in the EOG – electrooculography channel – and the flat tone of the muscle in the EMG – electromyography. 

The length of REM sleep periods will typically increase over the course of the night. This period is particularly important to witness when having a sleep study to titrate a CPAP machine or diagnose sleep apnea. 

Due to the muscle atonia and a lessened response to low oxygenation, apneas tend to be more plentiful and more significant during REM periods. The RPSGT will attempt to record time in the study in a supine position during REM sleep to ensure that the data collection is as complete as possible.

Sleep architecture is the pattern in which we move through these stages throughout the night. In a healthy sleeper, there are typically four to six sleep cycles each night. Each tends to be longer than the last, but on average they last for about 90-120 minutes each. 

During the cycle, the person falls asleep and drifts into N1 sleep. They will stay here for usually a very brief time before shifting into N2. N2 is typically the most abundant sleep stage. It is considered to be “light sleep,” and most people will spend about 50% of the night on the stage overall. In any given cycle, it typically lasts from 10 to 60 minutes. 

Next, the sleeper will go down into the N3 sleep, or deep sleep. The stage will typically last for 20-40 minutes though tends to get significantly shorter as you age. Some elderly people will have no N3 sleep at all. And lastly, the sleeper will go into the REM stage for 10-60 minutes.

When you have apnea, your brain thinks you are choking to death, and it is somewhat right about that. The airway is blocked, usually by your tongue, and no air is coming or going. Your brain spikes your heart rate, insulin, blood pressure, and anything it can to wake you up enough so that you will breathe. 

The fact that this happens is great because if it did not, you would have even more serious problems than sleep apnea. However, when this happens, it pulls you out of the later stages of sleep, such as N3 and REM. 

This creates sleep deprivation in those two states which affect your overall sleep quality and has a detrimental effect on your health – both physical and mental.

Having an apnea causes you to wake up in what we call an “EEG arousal.” You are not typically awake enough to be aware that you have woken up. You are just awake enough that your EEG activity shifts to the wake stage so that you regain muscle patency in your airway and begin to breathe again. 

These wake-ups cause your sleep to be highly fragmented and limit the amount of deep and REM sleep that you get throughout the night. When using a CPAP, the apneas are treated and therefore do not occur. 

This leaves you uninterrupted and able to shift through all the sleep stages in a normal cycle. As a result, you should feel more rested, but also you take a lot of strain off of your body systems so that you can live a longer and healthier life.

You can determine the quality of your sleep by taking a Home Sleep Test.