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 do it too. But trying to define it is a bit harder.

Sleep seems to be a restorative process. That part of the day (actually, in most species, several parts of the day) where the body regenerates, heals, learns – but it’s under intense study because we don’t really understand it.

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
• it is characterized by reduced or absent consciousness – 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 major phases of sleep by examining our brainwaves

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.

My sleep is all screwed up (I’ll show you later), so for now we’ll use a typical sleeper for this graph.

Sleep is basically 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 mostly where we dream, whether we can remember these dreams or not. It’s characterized by (wait for it) the rapid movement of the eyes (it’s further classified into tonic and phasic, but I don’t really think that matters here). REM normally occurs later in your sleep – towards morning, and generally occupies 20-25% of the total time you are asleep. During REM, your major 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 is timed differently (either no paralysis during REM or paralysis outside of REM), it presents interesting sleep problems (see this link about Sleep Paralysis from Hypnagognia).

A typical, average night

This graphic, from Zeo, shows a typical night of sleep. The light green represents REM. You can see how it occurs later in the sleep cycle, and becomes more frequent in the early morning.

The other broad category of sleep is Non-REM. This one used to be broken into four categories, 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 isn’t 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 or a device like a Zeo.

• N1 is what we also call Light Sleep. In the graph above, it’s the grey bars. This is the stage of sleep where we go from “restful consciousness” to the very beginning of a sleep cycle. You are easily awakened  - in fact people that are aroused from this state believe they’ve been awake all the time. If you’ve every 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 .. this is where we start to see “sleep spindles” and “K-complexes” in our EEG’s (subject of another post). N2 is identified by these EEG phenomenon. It’s also very easy 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 your 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 difficult than from N1, N2 and REM. If they are woken (like with an alarm clock) and they were in N3, they will be groggy and unfocused for quite a while…  This is how those “smart alarm clock” aps 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 really 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 suffer, 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…

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.

We all 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 changes even further.

It seems that the best judge of your sleep is you. How do you feel in the morning? Is 7 hours of sleep good for you, or do you need 9 to feel really rested? Can you easily 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, lethargic, 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 to actually sleep. Regardless, long-term sleep deprivation has some fairly serious health consequences – heart attacks and strokes among the worst.

So it’s in your best interest to address any deprivation issues you might have. It it’s insomnia, find out why (I’m in the middle of this stage right now). If it’s scheduling, find a way to fix it!

My Sleep – Your Mileage Will Vary

I told you at the beginning of this post that I’d show you how “untypical” I am – a textbook insomniac:

Last Night – Poor Sleep

But there’s a few interesting things here:

• I believed that I had no sleep at all – but I did (about 5:36 of it).
• I got N3 (deep sleep) the first part of the night. Not as much as I’d like, but 43 minutes of it anyways (note that an “average” amount of deep sleep for someone my age is 43 minutes, average REM is 1:54).
• My “sleep cycles” are all screwed up – there’s some bouncing around between deep and awake that may signify something – but I don’t know what.
• Finally there’s the number of awakenings (and their duration) at night. Mine were 14 last night, and typical is more like 4.

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

I’ll plan another post or two about the actual brainwaves – alpha, theta, delta waves, as well as sleep spindles and K-complexes. It’s interesting stuff, so I’ll put a link here when I do. (I did)

If you’d like to know much, much more about your sleep – I’d recommend a visit to Wikipedia.