The Structure of Sleep

What Sleep Is

Everyone knows what sleep is – we all do it. Every mammal and bird does it, and most reptiles, fish and amphibians. But trying to define it is a bit harder. Today, we will focus on sleep structure. Sleep seems to be a therapeutic process. That part of the day where the body regenerates learns – but it’s under intense study because we don’t understand its structure.
We do understand parts of it – the mechanics of it anyways. Here’s what we know so far:

  • it’s a natural state – we all do it, and many species in the animal kingdom do it too
  • reduced or absent consciousness characterized it – but we can arise from this state easier than if in a coma or in hibernation (which are also sleep like)
  • we have limited ability to react to stimulus
  • during some phases of sleep, our major muscles are “frozen.”
  • we can identify the significant stages of sleep by examining our brainwaves

It’s this last item where we’ve been able to “deconstruct” sleep structure into its major components. Today, I can have a look at the way I sleep through a brainwave monitor in my bedroom!

The Different Phases of Sleep

I’m only going to talk about mammals and birds here because we experience the same stages of sleep. Reptiles, fish, and amphibians are different.

Sleep structure is divided up into two broad categories (three, if you count being awake as one):

  1. REM or Rapid Eye Movement sleep
  2. NREM or Non-Rapid Eye Movement sleep

REM sleep is where we dream, whether we can remember these dreams or not. It’s characterized by the rapid movement of the eyes (it can be further classified into tonic and phasic, but I don’t think that matters here). REM occurs typically later in your sleep – towards morning, and generally occupies 20-25% of the total time you are asleep.

During REM, your significant muscles (arms, legs) are paralyzed or frozen – we’re not exactly sure why, but reason suggests it’s so we don’t “act out” our dreams. When this paralysis took time differently (either no disease during REM or paralysis outside of REM), it presents new sleep problems (see this link about Sleep Paralysis from Hypnagognia).

sleep structure

The other broad category of sleep is Non-REM. This one was in four groups, but now it’s generally stated as three (changed in 2007). When you see a reference to “N-1″ or “N-2″ sleep, the “N” just stands for “Non-REM.”

During any of the NREM stages, the body is not paralyzed, and there isn’t any rapid eye movement. Dreaming can happen, but it’s rare. Again, these stages are pretty easy to identify if the subject’s brainwaves are monitored with an EEG.

  • N1 is what we also call Light Sleep. In the graph above, it’s the grey bars. This is that stage of sleep where we go from “restful consciousness” to the very beginning of a sleep cycle. You are easily awakened people that are arousing from this state believe they’ve been awake all the time. If you’ve ever experienced sudden, jerky movement while you’re falling asleep – it’s this N1 stage you’re experiencing (and it’s completely normal).
  • N2 is also considered Light Sleep. It’s just a deeper stage of it. Here is where we start to see “sleep spindles” and “K-complexes” in our EEG’s (subject of another post). This EEG phenomenon identifies N2. It’s also effortless to be awakened in this stage, but dreaming is very rare during this phase.
  • N3 is what we call Deep Sleep. It used to be divided into N3 and N4, but it’s all grouped into N3 now. It’s the dark green bars in the graph. Sometimes this is also called slow-wave sleep, as the EEG recordings show a real slow-down of your brainwaves as you progress through the different stages (N1-N2-N3).
  • If you are going to dream outside of REM sleep, N3 is likely where it will occur. It’s during this phase where most parasomnias like sleepwalking, night terrors, and teeth grinding will occur. Waking a person up from N3 is much more complicated than from N1, N2, and REM. If they are woken (like with an alarm clock) and they were in N3, they will be tired and unfocused for quite a while. This is how those “smart alarm clock” apps work – they look for N3, and don’t wake you during it. They wait for you to be in another sleep state (and they can tell by your body motion).
  • N3 sleep seems to be important because your body will make sure that it gets this kind of sleep at the expense of any other. REM can suffer, and N1/N2 can endure, but you’ll always get the minimum N3 that you need. You’ll notice that the deep sleep – N3 – occurs early in the night.
  • The other category that we haven’t discussed is the red bars in the graph. It’s wakefulness – being awake. You’ll notice that this happens a few times a night – and this is very normal. As you go through a sleep cycle – typically around 90 minutes from N1-N2-N3-N2-REM – you’ll go through periods (usually a couple of minutes or less) where you are fully awake. Most of us will never remember it though.
sleep structure

How Much Sleep Do We Need?

Sleep is a very individual thing, so anyone that tells you that “you must get X hours of sleep every night for your health” is likely misinformed. All we can make are generalizations that may or may not apply to you and the structure.

We sleep less as we get older – some say this is a function of age, and some say this is a function of activity (mental and physical). We know that infants sleep a lot, and they have extraordinarily long REM sleep during their rest. As teenagers, it changes again. And as adults, it turns even further.

It seems that the best judge of your sleep is you. How do you feel in the morning? Are 7 hours of sleep good for you, or do you need 9 to feel rested? Can you comfortably sleep in an extra few hours on the weekend? It probably means that you could stand to get some extra sleep during the week!

Sleep Deprivation

Sleep deprivation is best described by how “normal” you feel the next day. Are you unfocused, lazy, uncoordinated? These can all be symptoms of not enough restful sleep. For some, it’s an actual medical problem – insomnia. For others, it’s a scheduling problem – not leaving yourself enough time actually to sleep. Regardless, long-term sleep deprivation has some reasonably severe health consequences – heart attacks and strokes among the worst. If you experience this, you should focus on sleep structure.

So it should be in your best interest to address any deprivation issues you might have. If It is insomnia, find out why. If it’s scheduling, find a way to fix it!

With all this, it goes to show that our bodies are remarkable machines. Also, we have learned about the sleep structure. No matter how messed up we make our sleep (intentionally or unintentionally) it finds a way to get what it needs.

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