Alright, so sleep is not just one continuous state. It's actually very dynamic and happens in cycles, and those cycles are called sleep cycles. So, a sleep cycle is basically going to be the movement through the different phases of sleep. Now, each sleep cycle will last approximately 90 to about 120 minutes and people will typically experience about 4 to 6 sleep cycles per night depending on how much sleep you are getting. Now, when we think about sleep, it's generally divided into two main phases.
The first phase is called REM sleep which stands for Rapid Eye Movement and it's exactly what it sounds like. So REM sleep is characterized by darting movements of the eyes. So, your eyes are closed, of course, but you basically have very rapid darting movements of the eyeball underneath your closed eyelid. And during REM, this is when we have very vivid dreaming. Now we can have some dreaming during the other phases of sleep, but most, like, our longest dreaming and our most vivid dreaming is associated with our REM sleep.
Some additional characteristics of REM sleep are that we are going to see an increased heart rate as well as increased respiration. So you're kind of in a state of, like, physiological arousal almost. However, we also see the paralysis of skeletal muscle. So, obviously, all of your vital organs are still perfectly functional. Your eye muscles are still working. Right? But your skeletal muscles, so your arms and your legs, for example, are actually unable to move when you are in REM sleep. And we think the purpose of that is to basically stop you from acting out your dreams, which could potentially be quite dangerous for you and people around you. Right? So during REM sleep, basically, our body is in a slight state of physiological arousal, but the majority of our body is unable to actually move.
Now our other phase of sleep is called nREM sleep, which literally just stands for non-Rapid Eye Movement sleep. And NREM sleep is divided into 4 stages. So these stages kind of vary between light sleep and deep sleep, and these stages are distinguished by their unique brainwave patterns, and we're going to talk about that coming up pretty soon. Some additional characteristics of nREM sleep are that we are going to see decreased heart rate as well as decreased respiration. So basically, everything kind of slows down in the body, and that is especially true as we get into those kind of like nice deep sleep states.
Now the way that all of this ties together is that each sleep cycle that we go through is going to include the nREM stages as well as a REM phase. So if we're looking at our little kind of circular graph here, you would basically enter right about here. You would enter stage 1 sleep, you would go into stage 2 sleep, and then into stages 3 and 4 sleep, and those would, of course, be our nREM stages. And then following that, you would enter REM, you would have a period of very vivid dreaming, and then you would cycle back into nREM stages. So again, each of these cycles would last between 90 and 120 minutes and you would have between 4 and 6 of them per night. Now as I mentioned, we distinguish between these nREM stages by looking at their brain wave patterns, and you may recall from chapter 2 that brain waves are visualized using something called an EEG, which is an electroencephalogram, and that is going to be measuring the electrical activity in the brain.
So coming up next for you, we're going to have a little EEG primer to help you actually understand how to read an EEG, and then we'll go over what those nREM stages actually look like. So I will see you there. Bye bye.