By now most of us have probably adjusted to the change in clocks. We are noticing the later hour that the sun is out and this might alleviate the grouchiness of the sleep rhythm adjustment. To be honest, it is kinda nice to be able to ride my bike home during some day light hours after work. And the perception of the day being longer leaves me feeling like I have a bit more time which I have been using to do some more reading about sleep, trying to understand it better.
Truly, this is not a simple feat for your average RMT; by no means am I a neuroscientist or sleep expert, and even they are still trying to sort out the odds and ends of sleep, why we do it, why we basically have virtually no choice in sleep, and what the consequences of sleep loss are. But before I get too far down that particular rabbit hole, first some basics.
It's a necessity
Like eating, sleep is a basic drive, a vegetative appetite that is apparently common to most species of animal. Recently I've even read articles that suggest plants may have a process not unlike sleep, following their own circadian rhythms (from science daily). So sleep is pretty universal. The currently accepted “definition” of sleep is a reversible behavioural state of perceptual disengagement consisting of motor rest requiring greater stimulus to around than when awake with easy reversibility. Generally, each species has it's own unique sleep posture and experience rebound increase after deprivation and circadian rhythmicity. Thankfully there are ethical standards to research but the non-negotiability of sleep has been established. Once upon a time, when scientific ethics were a little more scarce, extreme sleep deprivation could produce “psychosis or toxic delirium” ensued. So, to put it mildly, it's pretty important.
But neverminding that, the task at hand is to look at sleep itself, and in this article we will unpack a little bit of how sleep flows over the course of a night, and what exactly constitutes a good night's sleep.
Stages of Sleep
Chances are most of us are familiar with the concept of sleep stages. Loosely, we have Non-REM sleep (NREM) and REM sleep. NREM can be subdivided into “light sleep” and “deep sleep” and each of these can be further divided into stages N1-N4. Sleep cycles throughout the night almost like a wave pattern with gradual deepenings and risings through the stages, on a 90-120 minute interval. These intervals are referred to as cycles and within the cycle each stage occurs and is marked by changes in brain wave activity, measured by EEG.
Sleep begins at Stage 0, or wakefulness. In this stage, the eyes are closed but the person is still awake with alpha wave activity in the brain. Typically this stages lasts at most 15, maybe 20 minutes before the sleeper enters Stage 1. Stage 0 can be prolonged by opening the eyes or mental activity.
Stage 1/N1 sleep shows low-voltage, non-alpha wave activity with no REM. In this stage our muscular system begins to relax, our body temperature decreases and heart rate and respiratory rate also slow. Stage 2 /N2 shows high amplitude delta EEG waves, the muscles relax even further and HR and RR slow further. Together these two stages are considered “light sleep.” We spend most of our sleep time in N2 sleep; heart beat slows, breath rate slows and the muscles relax even further.
Stage 3 sleep is marked by 20-50% of delta wave activity and stage 4 is marked by >50% delta wave in a single epoch. In this deep sleep stage the body is completely relaxed and is necessary for that refreshed feeling when you wake up in the morning. Heart rate and breath rate slow to their lowest levels and brain wave activity slows further. A person in this level of sleep can be difficult to wake.
REM Sleep is noted by Rapid Eye Movements, muscle atonia (zero tension in muscles), middle ear muscle activity and EEG activity similar to stage 1 sleep. REM sleep can be broken down into two sub-parts; phasic which includes the eye movements, and tonic which has no eye movements but the same EEG activity. We do most of our dreaming in REM sleep and our bodies are actually paralyzed to prevent us from getting up and acting out our dreams. Our breath comes faster, HR picks up and we are at near wakefulness levels.
What is good or normal sleep?
A “Good Sleeper” experiences at least 95% efficiency, meaning of their time while in bed, 95% of it is spent sleeping. The sleeper experiences no more than 2 awakenings at night (ideally none), onset of sleep is swift, stage 2 takes up about 50% of sleep, REM is 20-25% of sleep and the remaining time is evenly distributed between stages 3 & 4. The sleep time spent in those deeper layers of sleep is what accounts for feel well rested in the morning. Generally, the first half of our night involves more time spent in stage 2 sleep, and the latter half of the night reflects more time spent in stages 3 & 4 and REM sleep.
Disrupted sleep That's what we are going to talk about next week. What can disrupt our sleep and does it differ when those disruptions happen at different points in the sleep cycle. By the end of all this, hopefully we will be a bit closer to understanding why sleep is important and how we can help ourselves improve our sleep if it is generally disrupted.
Flavie Waters et al. Severe sleep deprivation causes hallucinations and a gradual progression toward psychosis with increasing time awake. Front Psychiatry. July 2018 9: 303
Max Hirshkowitz, Ph.D DABSM, Normal human sleep: an overview. Med Clin N Am (2004) 88: 551–565
Lee K. Brown, Can sleep deprivation studies explain why human adults sleep? Curr Opin Pulm Med (2012) 18: 541–545