I’m sure you’ve been wondering all week about the strange ‘squiggles’ in my blog banner. Well, perhaps not, but the sleep geek in me hopes that at least one of you has. My blog banner has a segment of an electroencephalogram (EEG) or ‘brain waves’. This is a recording of my EEG, in fact. At this point in the night, I am in Stage 2 sleep. Each night when we go to sleep, we will experience non-REM sleep and REM sleep. Non-REM sleep (also written as NREM sleep) has been further categorised as Stages 1 through 4.
A bit of sleep history… The concept of sleep stages was first postulated in 1937 by Loomis et al. This concept went through several iterations in the following decades. However, it was Aserinky and Kleitman’s groundbreaking discovery of REM sleep in the early 1950s that set us on the path to defining sleep as NREM sleep and REM sleep. Sleep stages as we currently know them were defined by a committee of sleep researchers, chaired by Allan Rechtschaffen and Anthony Kales in 1968. These criteria have recently been updated by an American Academy of Sleep Medicine committee (2007) to comply with our current understanding of sleep physiology.
Most of us have an understanding that we go through each of the different sleep stages each night but few of us know what our sleep actually looks like. The answer is complicated as our sleep structure is different on every night, but sleep structure does follow a general pattern. Let me show you…
The diagram above shows a schematic hypnogram1 of sleep stages across the sleep period. A hypnogram is a graphical way of presenting changes in sleep structure. On the y-axis you will be able to see Stages 1 through 4 sleep and REM sleep. Stage 1 is the lightest stage of sleep and comprises up to 5% of the sleep period. Stages 3 and 4 are the deepest stages of sleep. These are collectively referred to as slow wave sleep2, denoting the slow frequency, high amplitude EEG seen during these stages. Our threshold for arousal is highest during slow wave sleep therefore this stage is also referred to as deep sleep. Slow wave sleep and REM sleep each comprise approximately 20% of the sleep period in a healthy young adult. The remainder of our sleep period is made up of Stage 2 sleep, or ‘light’ sleep.
The hypnogram also depicts changes in sleep that occur across the sleep period. Slow wave sleep predominates in the first half of the night and REM sleep in the latter half. Transition through all the sleep stages are reasonably cyclical and repeat every 90 minutes, approximately.
Does sleep really look like the schematic version of the hypnogram? Sort of…
These rapid changes in brain activity also mean it is also quite normal for the brain to return to wakefulness on a frequent basis. Arousals from sleep are shown in the lowest panel in the hypnogram. One vertical line represents one brief arousal from sleep. Arousals are an expected part of normal healthy sleep; in a healthy young adult these arousals typically occur 10-15 times per hour of sleep. We are not aware of any of these brief arousals as they usually only last a matter of seconds.
No doubt this post has left you with many questions. I can imagine a few…
Does our sleep change with age? Yes. Is there such a thing as ‘too many arousals’? Yes. Why is there a difference between the number of arousals and the indication of Wake on the hypnogram? We’ll have to leave the detailed answers to these questions for another time. If you have questions, let me know in the comments section and I’ll do my best to address them in future posts.
1 Hypnogram kindly reproduced with permission from Gander, P. H. (2003). Sleep in the 24-hour society. Lower Hutt, New Zealand: Open Polytechnic of New Zealand.
2 The 2007 sleep staging criteria refer to slow wave sleep collectively. That is, Stages 3 and 4 sleep are no longer defined separately.