Posted on 24 November 2022
Everyone wants to know the secret to staying healthy without putting in any effort. We are constantly told that no such solution exists. If we want to stay young and fit, we have to work for it – we have to exercise regularly and eat healthily. While this is certainly true, there is another pillar of health that involves literally zero effort, and is equally if not more important to us than diet and exercise. I am of course talking about sleep, the state in which we spend roughly a third of our lives.
When it comes to improving our health and wellbeing, sleep often takes a backseat relative to other strategies. This is perhaps because sleep is seen as something we don’t have that much control over – we get our sleep when we can, but for many people sleep has to fit between work, school, child rearing and other obligations. Despite this, taking a more active approach to optimising your sleep can be very beneficial and takes relatively little effort. In the first instalment of this series, we’ll explore what sleep actually is (so far as we currently understand it) and what sets good sleep and bad sleep apart. In part 2, we’ll cover how our sleeping patterns are controlled, as well as some of the environmental and genetic factors that determine how well we sleep. Finally, in part 3 we’ll discuss how you can leverage this knowledge in order optimise your own sleep, even if you struggle to get the recommended 8 hours a night.
There’s still a lot about sleep that we don’t understand, but it’s clear that sleep serves an important function, especially for the brain. All animals undergo some form of sleep, though scientists debate what the precise definition of sleep should include. Generally, sleep involves decreased activity of the nervous system and a suspension of consciousness. In humans and many other animals, sleep also involves muscle paralysis (at least during certain sleep stages). From an evolutionary perspective, sleep seems like a very bad idea: it makes the sleeping animal an easy target for predators, and takes up valuable time that could be spent hunting, gathering or mating. The apparent requirement for sleep in spite of these drawbacks doesn’t by itself prove that sleep is an evolutionary advantage. However, it’s hard to imagine why something so seemingly costly would be conserved in so many species if it didn’t serve some vital function.
If sleep does not serve an absolutely vital function, then it is the biggest mistake the evolutionary process has ever made.Professor Allan Rechtschaffen, pioneer in the field of sleep research
In humans and animals alike, the relationship between sleep and health is very clear. Rats deprived of sleep have severe health problems and die after a few weeks. In humans, lack of sleep is associated with increased risk of many chronic age-related diseases including heart disease, diabetes and Alzheimer’s disease, as well as poor mental health. This is most likely a two-way relationship as old age is also known to result in reduced sleep duration and quality, so below average sleep for your age group could be a consequence of ageing more rapidly than average.
The Function Of Sleep:
So, what is sleep, and why is it so essential for us? We once viewed sleep primarily as a period of inactivity, but that’s far from the truth. While the body may appear mostly inactive, the brain is anything but, and goes through multiple phases of activity throughout the night. We’ll discuss what these phases are and what they do later on, but in general it is thought that sleep is partly a way for the brain to restore itself after a day of activity, and partly a kind of ‘file transfer system’ for consolidating memories, experiences and skills recently acquired. Sleep is a time during which cells within the brain clear away harmful metabolic by-products that have accumulated during the day, and may also be involved in ridding the brain of faulty proteins such as amyloid. The effects of poor sleep quality and sleep deprivation on our cognitive performance are very plain to see, and most of us have experienced them to some extent.
The effects of sleep aren’t limited to the brain, though. Sleep involves changes in hormone secretion, a reduction of the metabolic rate, heart rate and blood pressure, and involves recovery and repair within many tissues including muscle. There’s increasing evidence that poor sleep disrupts the metabolism, altering the release of hormones that control appetite, as well as disrupting the way the body handles glucose (sugar), thereby increasing your risk of obesity and diabetes.
Because of the importance of sleep in so many aspects of our biology, good sleep should really be considered a central foundation of good health. Without good sleep, you aren’t going to have the metabolic health or the physical and mental stamina necessary to take full advantage of other healthy practices like exercise. But before we discuss how you can optimise your sleep, we first have to discuss what an optimal night of sleep actually looks like.
Most people know that adults should get between 7 and 9 hours of sleep per night. That’s not a bad place to start, but we can be more precise. Let’s take a look at what a typical night of quality sleep should look like.
Sleep is divided into two categories: rapid eye-movement sleep or REM sleep (named after the rapid eye movements that occur during this period), and non-REM sleep, which is itself broken down into 4 separate stages of increasingly deep sleep. As we drift off to sleep, the activity of the hindbrain (responsible for visual processing) is the first to decrease, followed by the rest of the brain. Sleep begins with stage 1 of non-REM sleep, the lightest stage of sleep, and transitions to stage 4 over the course of about 20 minutes.
If we measure someone’s brain activity during non-REM, we see that it slows down. By stage 4, brain waves occur only once or twice a second, but with a very large amplitude. What this means at a cellular level is that brain cells are not firing very frequently, but when they do, they do so in unison, with hundreds of thousands of neurons coordinating to become active simultaneously. Scientists think that the purpose of this stage is to transfer information across long distances within the brain. You can picture a large crowd of people trying to make themselves heard from as far away as possible. If everyone chants slowly and in unison, their words will remain distinguishable at great distances.
After about 70 minutes of non-REM sleep, the sleeper drops back down through stages 3 and 2 and into REM sleep. REM sleep is often viewed as the sleep stage with the most mystery surrounding it, as this is the stage during which most dreams occur. Brain activity during REM sleep starts to look more like waking activity, including an increase in activity of the visual system, while areas of the prefrontal cortex involved in decision making are suppressed. This is thought to be why we experience visual information during our dreams, but are usually unable to recognise that we are in a dream or to influence them in any way. Lucid dreams are the obvious exception to this, and can be made significantly more likely using techniques designed to make the prefrontal cortex more active during REM sleep.
The purpose of REM sleep is uncertain. It probably plays a role in memory consolidation and function. Another interesting theory is that it exists to stop the visual cortex getting ‘taken over’ by other brain regions. When the brain loses sensory input, for example when a limb is amputated, the neurons involved can be remapped to other areas. This is why amputees sometimes experience sensations in their missing limb when another area of the body is touched. This rewiring can start happening quite quickly if no sensory information is being processed. Some scientists wonder whether REM sleep exists to keep the visual system active during the night so that such rewiring does not occur.
Let’s now return to the topic of what makes good sleep. Good quality sleep should include a balance of the different sleep phases. If you take another look at the graph showing the sleep cycle throughout the night, you may notice that the 4 stages of sleep aren’t evenly distributed. During the first sleep cycle, only a brief period of time is spent in REM sleep, as most REM sleep is occurring during the last few cycles. This means that if you lose a few hours of sleep, you aren’t just losing 25% of your sleep – you could also lose over half of your REM sleep for the night. Other factors such as caffeine consumption can also affect how much time you spend in different sleep stages.
Good sleep should also be continuous and uninterrupted. It’s normal to briefly wake in-between two sleep cycles, and these periods of wakefulness are often forgotten by morning. Waking up for longer periods of time (especially in the middle of a sleep cycle) is problematic. It not only reduces total sleep time, but also interferes with the progression of the sleep cycles throughout the night. Research suggests that poor sleep continuity leads to worse cognitive performance in both younger and older age groups.
The third key to good sleep is timing. We’ll discuss this in a little more detail in part 2, in which we’ll cover how sleep is controlled. Our bodies go through a 24-hour cycle called a circadian rhythm, which is controlled primarily in response to light. This sleep-wake cycle promotes alertness during the day and promotes sleep at night through chemical signals. Attempting to sleep during the waking phase of the sleep-wake cycle leads to poor quality sleep. This is why shift work is so unhealthy – unless the shift worker experiences very bright light during their shift and no light at all during the day, they will be unable to attain the full benefits of sleep.
That concludes our rather lengthy first instalment to this series. Hopefully you now have a better understanding of what sleep is for and why not all sleep is created equal. In the next part, we’ll be taking a look at how our sleeping patterns are controlled, and how our genes and our environment can affect how much sleep we get and when we should get it.
Sleep deprivation in the rat: III. Total sleep deprivation: https://doi.org/10.1093/sleep/12.1.13
Sleep Disorders and Sleep Deprivation: An Unmet Public Health Problem.: https://www.ncbi.nlm.nih.gov/books/NBK19961/
CONTROL OF SLEEP AND WAKEFULNESS: https://doi.org/10.1152%2Fphysrev.00032.2011