In Hong Kong, sleep is something that we (hopefully) do each night, but how often do we actively think about the process? In my case, I am ashamed to admit that the answer used to be “not enough”, but I have since done a lot of reading on the subject and want to pass some of this info along to fellow Hong Kong residents looking to learn more about sleep.

It’s easy to think of sleep as one continuous process. In our conscious perspective, we go from being awake, to sleeping, to waking up again. It seems simple enough, but the reality is quite the opposite. Sleep is actually not one homogenous state, but a progression through five unique stages, each distinguishable from one another by their unique characteristics. It is worth noting that some contemporary sleep scientists now combine two of the stages (stage three and four), so the total number can also be listed as four.

With this post, I’d like to take a look at all the various things that happen when we shut our eyes for a good night’s sleep.

First things first, as some of you may already know, we can divide sleep into two main behavioral states. The first is rapid eye movement, commonly known as REM, and the second is the uninterestingly named non-REM. REM is a stage of sleep in and of itself, while non-REM can be further divided into 4 sub-stages, which gives us our total of five. The four non-REM sub-stages are differentiated by their EEG levels, which looks at the bioelectric activity of the brain. As we advance further through the stages of non-REM sleep, our sleep becomes deeper, so stage one is the lightest sleep and stage four the deepest. Typically, each night we have about 75% non-REM sleep (stages 1-4) and 25% REM sleep.

Personally, like most people, I find the most interesting stage of sleep to be the REM stage, as this is when our dreams occur. As mentioned earlier, REM stands for rapid eye movement, and in the REM stage of sleep, our eyes move as the brain attempts to view the events in the dream world. For example, if we look to our left in our dream, under our closed eyelids our eyes will be looking left as well. This very cool process is known as the scanning hypothesis. In many ways, these eye movements are no different than our eye movements when awake – they are characterized by darting movements and binocular synchronicity (both eyes move in unison). However, during REM, our eyes do not center themselves as they would during wakefulness and instead occasionally perform drifting movements.

During REM sleep, the brain is highly active. When awake, we perceive and react to the things around us, but while asleep, the brain both creates the world then perceives and reacts to it. Overall, the amount of brain activity during REM sleep is still quite similar to brain activity when awake though, with rapid brainwaves existing in both states. In some cases, REM can spill over to wakefulness in a process known as ASP (awareness during sleep paralysis). This typically happens to people at least once in a lifetime and can be quite a shock. In simple terms, sleep scientists have defined REM sleep as being like a highly active brain in a paralyzed body.

Non-REM sleep is not quite as exciting as REM sleep, but it is still an important and interesting process. Unlike REM sleep, we are not fully paralyzed during the non-REM stages, and frequently have low level movements of the neck and jaw muscles. Sleepwalking occurs during non-REM sleep, though overall this is not common. Throughout the night, our bodies progress in and out of the four non-REM stages.

Stage 1 operates as something of a transition period between wakefulness and the other stages of sleep. It is typically short, lasting only 1-7 minutes, and is characterized by low voltage, mixed frequency brain activity. Stage 1 is the easiest sleep to interrupt. Stage 2 provides the bulk of our nightly sleep, taking up around 40-45%. Stage 2 is characterized by a ‘sleep spindle’ in our EEG scans, making it easily identifiable from the other stages. Stage 3 and Stage 4 are both similar in function and, as mentioned earlier, are sometimes grouped together as one stage. They are both what is known as slow wave sleep, and contain high amplitude waves in EEG readings. Stage 4 is typically the hardest to wake up from.

Together, these stages combine to make cycles, which typically last about ninety minutes and rotate through the various stages. Early in our sleep, non-REM sleep is dominated by stages 3 and 4, which makes it difficult for us to wake up. As we go further into our sleep cycles, the REM stages last longer and but the non-REM is predominately stage 2.

Overall, it is amazing how much work our brains do, even after we have clocked out for the day!