What is sleep all about?

By Maria M. Hadjimarkou, PhD - Lecturer School of Psychology University of Sussex

The birds do it, the flies do it and we do it. We all sleep. In fact, several species of living things, even bacteria, also sleep. So, what is sleep? Sleep is a natural, periodic state during which our movement and responsiveness to our environment are greatly reduced. It is periodic, because it takes place at regular intervals, i.e., every night, and it is natural because it takes place naturally, without us having to work hard or do something specific to induce it. Usually, we just need to settle in a comfortable environment, such as our bedroom, where we know we will lie down for a few hours and sleep until the next morning. Although sleep seems like a simple thing to do, it is not as easy to study and understand. Sleep is currently one of the most exciting and rapidly developing areas of research, as we still try to unravel its secrets. As you will find out, sleep is important not only because it helps us rest so that we can carry on the following day, but also because during sleep, several key processes are taking place with important implications for our physical and mental health, our ability to learn and remember, and much more.

My name is Maria Hadjimarkou and I am a lecturer at the University of Sussex, in the School of Psychology. I would like to share with you some of this knowledge and important discoveries in the area of sleep research, in hope that you will be able to learn and appreciate it as much as I have. There are several ways in which we can study sleep. We can ask people about their sleep habits and their quality of sleep using questionnaires. We can ask them to keep a diary or use an app to collect information about their sleep and wake. However, the gold standard for studying sleep in both humans and animals is the electroencephalogram (EEG), discovered by Hans Berger in 1929. This approach involves the placement of electrodes on the scalp (a non-invasive way) that pick up electrical signals from the neurons in our brain. When a person is awake, the signals are fast and asynchronous (beta waves), especially when processing information or they can be slower and more synchronous when one is awake but relaxed (alpha waves). When we go to sleep, contrary to expectations, our brains do not turn off. Similarly, they do not just slow down until the next morning when they can start to speed up again. It turns out that our sleep is not a monotonous state where our brain is just suspending all functions until we wake up again, but it goes into a different mode which involves several distinct patterns of activity, known as sleep architecture. These different patterns of activity are taking place in a particular order and they tend to repeat or cycle until sleep is complete.

There is consensus among experts that there are three distinct stages that take place at the beginning of the night, stages 1, 2 and 3 where the brain is indeed moving towards a deeper sleep and its activity is progressively getting slower so that by stage 3, the waves picked up by the electrodes on the scalp are slow and synchronous and are thus called slow waves (delta waves). It is during this stage of slow-wave sleep that it may be difficult to be awoken and if we are awoken, we may need a minute or two to become alert. However, this deep sleep is not the end point of our sleep journey during the night. After spending a few minutes in this stage, the activity of the brain starts to pick up again and we cycle back to stages 2 and 1 and then surprisingly, we enter a completely different stage of sleep, a fourth stage, called rapid-eye-movement sleep. This is a stage of shallow sleep associated with a high degree of brain activity which is comparable to wakefulness. Paradoxically, even though we are asleep, our eyes start to move vigorously, thus the term ‘rapid eye movement’ or REM sleep, and our bodies become paralyzed. You may be aware that this distinct stage of sleep, REM sleep, is associated with dreaming, although it is now established that dreaming can occur throughout the night and during all stages of sleep.

The different stages of non-REM (1, 2 and 3) and REM sleep repeat every 90 minutes, so we tend to have a few of these cycles every night. However, the cycles do not repeat in the exact same way across the night. There are some differences when we compare the recordings from the earlier part of the night versus the later part of the night. In the early part of the night, the first few hours after going to sleep, the sleeper spends more time in the deeper stages of sleep, in stages 2 and 3, and less time in the more shallow stages of 1 and REM, whereas in the latter part of the night, the opposite happens (see figure for an example of a typical night). The sleeper tends to spend less time in the deeper stages of sleep and has longer REM episodes as the night progresses; you may remember waking up in the morning while in a dream. Those of you with children probably know that you can be noisy during the first part of the night without disturbing them, whereas in the morning even the slightest noise can wake them up. 

Is this pattern of activity in the sleeping brain important in some way? What is the point of cycling through different stages every night and why do we spend more time in deep sleep earlier in the night and more time in shallow sleep later in the night? Well, it turns out that these different stages of sleep serve different functions, even though not all of them are fully understood. In the subsequent entries, I will be sharing with you the most important discoveries in sleep research. I hope that you will find them both exciting and intriguing.  

Reference:
Carlson N. R & Birkett, M. A. (2017). Physiology of Behavior, 12th Ed., Pearson.