What is Sleep?
“Sleep is not just simply the absence of wakefulness” – Matthew Walker.
It is a deliberate, highly metabolically active state of the brain and body which is made up of distinct stages.
These stages are broadly differentiated by one core feature, the movement of the eyes. Thus, sleep is majorly broken into NREM sleep (non-rapid eye movement) and REM sleep (rapid eye movement).
Sleep is essential for both physical and mental health. In some form or another, sleep has been discovered in almost every living organism. If it were not absolutely critical, it seems only logical that we would have got rid of sleep a long time ago, as it reflects a great amount of time each day when we are at our most vulnerable. We are limp, still and unresponsive.
Instead, it is a non-negotiable factor of life that allows each and every system in our body to be refreshed, rejuvenated, and replenished so that upon awakening we are ready to tackle the day ahead.
The Stages of Sleep
NREM sleep is characterised by:
- Eye movements: None
- Muscle tone: Some – less than waking state
- Brain waves: Deep, slow, and synchronised – rippling from the medial frontal lobes to the back of the brain
NREM Sleep is further divided in 4 stages. Each stage is characterised by slightly different brain wave activity and most significantly, increasing difficulty to wake up from each deeper stage. As such, it is more difficult to wake someone from stage 4 of deep NREM sleep than it is from stage 1 or 2 of light NREM sleep.
REM sleep is characterised by:
- Eye movements: Rapid, irregular, side to side movements known has horizontal saccades
- Muscle tone: None – complete paralysis
- Brain waves: Shallow, rapid and CHAOTIC – even more active than waking state in some instances
REM Sleep is the stage in which we experience vivid dreams and in terms of brain activity is the most similar to the waking state.
The Sleep Cycle
As we have learned, sleep does not simply consist of one long span of unconsciousness. Sleep is divided into different stages based on varying degrees of brain activity, eye movement and muscle tone.
Throughout the night, sleep is not just divided equally into these stages, it is beautifully and intricately organised into the sleep cycle. On average, a single sleep cycle lasts roughly 90 minutes, where we cycle between the various NREM and REM stages. This means that during an 8 hour sleep window you will complete just over 5 sleep cycles. However, during each sleep cycle the distribution of Light NREM, Deep NREM and REM stages are NOT equal.
Think of one 90 minute sleep cycle like a valley. On either side of the valley, at the top of the ridge you have the waking state and at the bottom you have the deepest stages of NREM sleep. As you descend into the valley to reach the other side, you encounter as series of ledges, with each representing a stage of sleep. As you continue to descend deeper into the valley, you descend deeper into the stages of NREM sleep. Eventually, on your journey you will reach the bottom and start your ascent out of the valley, climbing your way back towards consciousness. As you climb, you will pass each of the levels associated with lighter stages of NREM sleep until you reach the final ledge before the top, REM sleep.
This analogy works well to describe the fall and rise of the sleep stages throughout a single 90 minute sleep cycle, however unlike a single valley, the sleep cycle is not the same with each repetition. Throughout the night the amounts of NREM and REM sleep present in each cycle vary. In this way, the total sleep experience is like a series of valleys, each with different depths, number of ledges and varying heights at which these ledges are found, where the goal is not the next ridge but traversing the whole series of valleys back to the waking state.
In general, the ratio of NREM to REM sleep in adults is roughly 80/20, meaning that during a typical night of sleep, we spend more time in the valley than we do closer to the ridge line. Also, Deep NREM sleep is more prevalent at the start of the night, while Light NREM and REM sleep are more prevalent at the end of the night. In regards to our analogy, we traverse deeper valleys at the beginning of the night and shallower valleys towards the end of the night spending more time closer to the ridge line.
NREM & REM Sleep for Mental Health
All stages of sleep are important and it is the full spectrum of sleep that allows us to wake each and every day ready to tackle the days obstacles, however there appears to be specific functions of the individual stages that are crucial for mental health.
Sleep loss, in general, impairs the prefrontal cortex, the CEO of the brain, which functions to control logical reasoning, decision making and problem solving.
Normally this brain region puts a brake on deeper, more emotional centres of the brain that regulate our emotions such as fear, anger, impulsivity, and reward. As a result, the sleep deprived brain tends to swing more violently to both emotional extremes. Therefore, when we sleep less it is more common for us to quickly shift from feeling happy and elated to feeling angry and frustrated and for us to experience emotional outbursts or irrational thoughts.
NREM sleep, especially that which occurs early in the night, plays a critical role in mental and physical rejuvenation and supports cognitive health. It is the deepest stages, 3 & 4, that protect us from the cognitive decline associated with Alzheimer’s Disease, which can be profoundly debilitating. Studies have demonstrated that individuals who suffer from Alzheimer’s experience a more exaggerated loss of sleep compared to their normal age related counterparts and that this disruption of sleep precedes the cognitive changes by many years.
It is during the deepest stages of sleep when the glymphatic system, the brains waste removal system, is most active and clears the debris, metabolites, and other molecules that may build up and contribute to poisoning the cells of the brain.
REM sleep is also vitally important for mental health. It is the stage of sleep when we experience vivid dreaming. The current state of the academic literature suggests that dreaming and REM sleep have 3 main functions; overnight therapy, decoding of emotional experiences, and promoting creativity and problem-solving.
When we begin dreaming, studies have demonstrated that four main brain regions become highly active; visuospatial regions (vision and spatial awareness), motor regions (generation of movement), memory regions (hippocampus associated with short-term memory), and emotional regions (amygdala and cingulate cortex associated with fear and reward). At the same time another brain region becomes deactivated, the lateral prefrontal cortex (associated with rational thought and decision making) and there is a complete absence of noradrenaline, a key stress related chemical in the brain.
These changes in brain activity during REM sleep create a unique chemical environment in the brain that acts to take “the painful sting out of difficult, even traumatic, emotional episodes you have experienced during the day or previous days, offering emotional resolution when you awake the next morning” (Matthew Walker).
Essentially, we get to re-experience previous emotional themes and memories in a brain state without the stress chemical that links our physiological stress response to these memories. As a result, dreaming offers a form of emotional therapy where we can divorce the emotional charge associated with previous memories.
Furthermore, this brain state during REM sleep appears to be critical in the recalibration of our emotional centres for the next day so that we do not bounce from one emotional extreme to the other. Studies have shown that a lack of sleep leads people to a greater bias for fear and causes them to judge pleasant and gentle facial expressions as more threatening and menacing.
Simple Tips to Improve Your Sleep
If you have read this far, you have probably gained a whole new appreciation for the intricate and powerful nature of sleep. When I took the time to learn about what sleep was really doing for our physical and mental health, it caused me to change my attitude from ‘I’ll sleep when I die’ to ‘If I don’t sleep, I’ll die.’
In order to take control of your sleep there are some simple behavioural and lifestyle tips that you can do today to learn how to sleep better. Just as our habits create the reality of our life, our habits can also either set us up for a good night’s sleep or they can lead us to a bad night’s sleep.
The best tips backed by strong science include:
- 10-30 minutes of morning sunlight
- Limit alcohol and consume caffeine before 12pm
- Devices on night mode and dim household lights
- No food or drink within 2-3 hours before bed
- Have a hot shower or bath 1 hour before bed
- Make your room cool and dark at night
- Try a breathing practice before bed
Give these habits a try for at least 1-2 weeks and notice the difference for yourself. If you want to learn more about the science of why these behavioural and lifestyle tips work, check out one of our previous blogs, How to Sleep Better: 7 Behavioural and Lifestyle Tools
References
- Why We Sleep: Unlocking the Power of Sleep and Dreams by Matthew Walker
- Master Your Sleep and Be More Alert When Awake Episode – Huberman Lab Podcast
- Dr. Matthew Walker: The Science & Practice of Perfecting Your Sleep – Huberman Lab Podcast
Disclaimer: The information provided in this blog post is for educational purposes only and is not intended to be a substitute for professional medical advice, diagnosis, or treatment. It is important to consult a qualified healthcare provider before starting any new health regimen, particularly if you have any medical conditions, injuries, or concerns. The author and publisher of this post are not responsible for any adverse effects or consequences that may arise from following the information in this post without appropriate professional medical supervision.