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How much sleep do you need?

woman thinking about sleep

N.B. Please don’t take this as medical advice. It ain’t.

The question of how much sleep you need has always struck me as hugely important. We’re all trying to live happy, productive lives, and yet we spend about a third of our time at zero productivity or enjoyment. What’s up with that?

I’ll admit that for a short period of time, Matthew Walker’s bestselling book Why We Sleep felt like the answer to that question. Walker asserts that sleeping six hours a night or less is linked to everything from looking less attractive to cancer. He also implies that anything less than eight hours is likely to be doing us at least some harm.

Convinced of the case – after all, he cites lots of scientific studies in the book – I finally made a conscious effort to break my crappy sleeping habits. I’m sure like many people, I used to go to bed when I felt tired, get up just in time for work, and make do with whatever energy that gave me each day.

For a few months, I stuck to a regular sleep schedule, religiously tracked the amount of time I slept, and took to believing that any alcohol I consumed was likely to have destroyed any learning I did that day, as well as the next day’s productivity.

The downside of sleeping more

In the short term, these were not at all bad – not least the reduced drinking. But I was spending between one and one-and-a-half hours more per night in bed. Was the extra sleep worth shaving more than 22,000 hours, or 935 days, off the rest of my life1?

Thankfully before writing an embarrassing article about all the ways Why We Sleep was going to improve my life and yours, I started looking into the research behind Walker’s claims. Quite early in the search I found this devastating critique by Alexey Guzey. Guzey’s post is worth reading in full – or there’s a good distillation of his points in episode 344 of the Smart People Podcast. My short version is that Why We Sleep relies on poor data, distorts that data to make it seem more powerful, and dresses it in the cloak of academic literature to make it (and Walker) seem more impressive.

The first obvious implication is that I should be more careful about how I update my beliefs and practices based on stuff I read! It’s a helpful reminder that before making a big change (such as one that will affect 1/16th of the time you’re alive), it’s worth checking the facts. After all, extraordinary claims require extraordinary evidence. You could argue that I’m falling foul of the same thing in believing everything in Guzey’s post. But the point here is not that Guzey has all the right answers, but that Why We Sleep doesn’t either.

We don’t know how much sleep is optimal

The question remains, of course, as to how much sleep you do need. Guzey points to a meta-analysis of studies looking at the relationship between sleep and all-cause mortality2, adjusts for people’s overestimation of the amount of sleep they get, and ends up at a figure of six hours per night. He does point out, though, that this is based on correlation rather than causation. His own conclusion is that generally speaking:

“We have literally no idea about the optimal for long-term health sleep duration.”

My own search for a systematic review of methodologically-sound randomised controlled trials testing the long-run effect of different amounts of sleep turned up…nothing. This review in Nature and Science of Sleep puts it nicely:

“In summary, there is no magic number or ideal amount of sleep to get each night that could apply broadly to all. The optimal amount of sleep should be individualized, as it depends on many factors.”

But even if we don’t appear to have a good answer about the optimal amount of sleep for long-term health, it seems uncontroversial to say we need some of it. Like eating, security, and sex, we have strong urges towards sleep, even if we don’t know why it’s evolutionarily adaptive.

So what’s a reasonable amount to sleep?

The US National Sleep Foundation’s 2015 recommendations of seven to nine hours seem a reasonable place to start. If nothing else, the authors’ combined ‘guesses’ are more likely to be closer to the truth than a single study.

In trying to best ‘individualize’ how much you sleep, taking seven to nine hours as your baseline, it seems reasonable to consider the following factors:

Factors to consider

  • What are the goals you’re seeking to achieve? Do you want to minimise the time you’re asleep, so you can be doing other things? Or would you rather sacrifice more time to feel less drowsy during the day?
  • Do you have other factors influencing the length of time you sleep, such as a partner, children, or a job requiring exceptionally long hours?
  • How efficient are you? Do you need an eight hour window to sleep for six? Or do you get straight to sleep and wake up little during the night?
  • Do you need other means to boost your wakefulness during the day, such as caffeine or napping? What are the costs and benefits of those things?
  • Do your daily activities require you to be alert at all times? Or can you afford to have periods of drowsiness, or even micro-sleeps? Are you able to organise your day around those periods?
  • Are you using other available means of enhancing your wakefulness, like reasonable diet and exercise?

Can we even change how much we sleep?

This is a lot to consider here, and the extent to which you’re able to change your habits will vary from person to person. For some, paying to have a professional run a load of tests on you, and investing in the most expensive mattress will be worthwhile. For others, getting through the night without being woken every five minutes by a screaming baby is a victory. But given the huge amount of time we spend sleeping, most people could benefit from taking a more deliberate approach.

For me, that will mean continuing to track my ‘sleep opportunity’ time and my ‘estimated sleep’ time through my Sleep Cycle app. I also keep a diary of how wakeful I felt throughout the day, and how it seems to have affected my productivity and wellbeing. Over time I should build up enough knowledge to find out how much sleep I need personally.

Of course, how much sleep you need is only one part of the equation. It also makes sense to try to maximise the quality of our sleep as far as possible. This list from the Sleep Association is a decent place to start.

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P.S. I’ve also done a YouTube video version of this post.

Footnotes

  1. based on 1.5h per night x 365 x the number of years I have left to live, according to this life expectancy data. That seems quite conservative – if I only live till 73, I’ll be pretty disappointed!
  2. not the only factor to consider, of course