As I start to think about this question, I can feel the inner scientist in me asking a range of questions in response to your first. Sadly, I have a limit to how long my response should be before people start turning off… So, to prevent his becoming a short thesis in itself (or several), I have opted to approach this question in terms of a healthy adult and to focus on how much sleep we need to remain physically and mentally healthy, and not just simply survive.
It is important to ask how much sleep we really need, and just as important to make sure we actually try to achieve this. As we’ll see, sleep is vital for the healthy functioning of many physiological and mental faculties that to skimp on it is inexcusable. As I type this, I can hear many crying out “modern living”, “deadlines”, “fitting everything in”, and even the faint whispers of “YOLO” on the breeze. However, sleep is not simply an inconvenience to our daily lives but an important part of keeping us functioning.
To highlight this, let’s look at the effects of a lack of sleep. A chronic lack of sleep has been shown to be associated with heart disease, type 2 diabetes, and stroke to name but a few. It should also be noted that there’s such a thing as ‘too much’ of a good thing, as excess sleep is associated with similar problems as too little sleep (Shen et al., 2016). At some point, you have probably felt the effects of lying in for too long and the scourge of ‘sleep hangovers’.
How do we measure an ‘optimum’?
There are a number of ways we can do this, but first we need to understand what we mean by optimum and why we sleep in the first place. The exact function of sleep is still not fully understood but it is generally agreed that sleep enables us to grow, is vital for memory and keeps us alert and healthy. In fact, prolonged durations without sleep can distort our perception of reality, interfere with our memory in a staggering way and, in very extreme cases, lead to death.
By looking at this in the long-term, we can identify the ‘optimum’ amount of sleep by observing lots and people’s sleep habits and to identify the risks associated with differing hours of sleep. We can also deprive individuals of sleep and see how they function after a couple of hours (or complete) sleep restriction compared to a normal night. For example, if we deprive someone of 1 or 2 hours sleep per night do we see any effect of an individual’s ability to function during the daytime? If the answer is yes, then this would suggest that those extra hours of sleep are important to us in some way.
So, we are now more familiar with why we should attempt to get the right amount of sleep, and how to measure the importance of sleep, but how much do we actually need? The simple answer is that 7-8 hours is generally considered to be ideal for the majority of individuals. A recent study highlighted that 7 hours was optimum for avoiding the negative impacts of too little or too much sleep (Shen et al., 2016). Another comprehensive review highlighted again that between 7-8 hours was optimum when considering all-cause mortality (i.e. your chance of death is reduced if you get between 7-8 hours of sleep a night, when compared to more or less sleep).
What about those who can survive with less than 7 hours?
However, although there is some merit in this claim, it has created the idea that there is a ‘perfect’ amount of sleep, and that this is the same for everyone. This really is not the case and the amount of sleep an individual ultimately needs to feel refreshed and to function varies from person to person. To many sleep scientists this is source of frustration and avid fascination. Take, for example, individuals who only need 5-6 hours of sleep a night to function, and those who claim they can get by with even less.
Why might it be that some people need less sleep than others? The simple answer seems to be that it all lies in our genetics, and certain mutations in our genetic code are linked to a greater resistance to the effects of sleep deprivation. A particular set of gene mutations outlined by Pellegrino and colleagues (2014) was associated with a reduced need for sleep and fewer negative effects associated with reduced sleep (6 hours). They established that a reduced need for sleep, or an increased resistance to sleep loss, is heritable and the genes involved seem to impact the internal clocks we have in every one of cells.
Yet, there is a problem with these studies, and others which have looked at those resistant to sleep deprivation. Some individuals may be resistant to effects of sleep deprivation on attention and day-to-day functioning, but most of the large sleep studies have focused on the health impacts of sub-par sleep over very long periods of time. These ‘sleep-deprivation resistant’ studies are conducted over the course of a couple of weeks; the long-term impact of sleeping less than 7-8 hours, and being resistant to short term sleep restriction, has not be extensively studied.
Therefore, it is conceivable that individuals can go with less than 7-8 hours’ sleep in the short term, and some may be better than others at getting away with this. However, ‘getting away with it’ may be just the term we want to use here. These individuals may be able to compensate early on but still be prone to the same health issues associated with reduced sleep as the rest of us in the long term.
Effect of Chronotype
I am about to go off a seemingly unrelated tangent but bear with me (it’ll be interesting, promise!) When people think and talk about sleep in terms of what is optimum and healthy, they will often focus on the duration of sleep. Most people will also mention when they drop off and wake up, but a large part of this conversation will be in relation to their willpower and how they “should really go to bed earlier and wake up earlier”. However, when we go to sleep is an important factor to keep in mind as we look at what healthy sleep looks like. Each of us can be classified on the basis of something called a chronotype and this describes at what time we go to sleep and wake up on a typical day. Although people fall along a complete spectrum, and show some variability, there are two main camps which most tend to fall into: early risers and night-owls. As the names suggest, early risers go to bed earlier and wake up earlier than the night-owls who find themselves more productive later in the day and subsequently wake up later. Great, thanks for the information Jack, but why is this important for my further understanding of healthy sleep?
Although sleep duration is important for a healthy mind and body, the time at which our body wants us to sleep and rise will impact on whether we can achieve the right amount of sleep in the first place. Our current schooling and working world favours the early-risers. As a result, night-owls unfortunately have to constrain their normal sleeping patterns during the working week to fit with the demands of a 9-5 society. This is important, as night-owls may catch-up slightly on their sleep schedules during the weekend but this is rarely enough and produces what is known as social jet-lag. This is the mismatch between what our body-clocks are screaming at us during the week when we wake up earlier than we would naturally do so, and the extended sleep we have during the weekend to attempt to rectify this.
The attempt to rectify the sleep-debt during the weekend is not enough, and this is highlighted by the increased prevalence of mental illnesses in those who score as night-owls on chronotype measures. Therefore, when as well the duration of sleep is important when we consider what healthy sleep should look like. Granted, altering the duration of sleep might be simpler to achieve for most of us than being able to get up later, but that does not make ignoring our own sleep rhythms any less important.
So, although I could likely go on for much longer, I should leave it there. The take home message is that 7-8 hours is the ideal duration of sleep required for a healthy existence. We should also be mindful that duration is not the only indicator of healthy sleep, and listening to your own sleep rhythms is important to ensure you get sufficient and good quality sleep.
So, I have to admit that the inspiration for this question came not from me directly. In fact, the question was part of a project which is collecting an increasingly large database of questions about science in general. The project, known as The Science Room, is run by a good friend of mine in Southampton, and it is determined to answer everyone’s science questions (ambitious, I know!) On that note, if you’re interested in asking any science based questions or learning more about the project, you can find the current webpage here: http://thearthousesouthampton.org/the-science-room/
The answer to this particular question is due to appear on a website dedicated to answering all of these questions (amongst many other things!) I shall link that site here when it’s complete.
Header Image: Exhaustion
Second Image: Clock