We are often warned about the health problems associated with undersleeping, including higher mortality rates among those who don’t get enough sleep. But it’s rarely pointed out that chronic oversleeping can cut years from your life too — in a manner of speaking. 30 minutes in a dreamless sleep are 30 minutes of your life that you will never see again. Oversleeping makes your life shorter than it needs to be, and we ought to do more to discourage it.

How oversleeping shortens your life

Usually, when we say an activity "cuts years from peoples’ lives," we mean that it causes people to die earlier than they otherwise would. If someone who would normally have lived to be 70 dies from lung cancer at 60 because they were a heavy smoker, then we conclude that smoking cut ten years from their life. But do we care about maximizing the number of years between the date of our birth and the date of our death? Or do we care about how many years of life we get to experience while we’re still around? I think it’s the latter and not the former that we really care about.

To see why, imagine that you're given the following choice: you can either live for another 30 years, or you can spend the next 30 years in a dreamless coma before waking up suddenly and dying a single year later. Wouldn’t you choose 30 years of lived experience, even if it means your lifespan is one year shorter? I know I would. In fact, I would opt for a single day of lived experience over one hundred years spent in that dreamless coma.

So we don't really care about the number of years in which our hearts are beating and our lungs are inflating. We care about the number of years in which we are experiencing life. And if we care about years of lived experience, then the time we spend in unnecessary dreamless sleep is time that has been quite literally cut from our life. In other words, a year spent in unnecessary dreamless sleep over the course of your life is no better than a year lost to premature death.

Sleeping more than you need isn't good for you

I’m not advocating undersleeping, by any means. Everyone should get the sleep they require. But the amount of life lost to unnecessary sleep could be significant. Someone who takes 30 minutes of unnecessary sleep each day between the ages of 20 and 70 should expect to lose over a year of life. An OECD report indicates that the average sleeping time in China is just over 9 hours per night. If this average could be safely reduced to 8 hours per night, it would prevent over 150,000 years of lost life each year.

One possible objection is that sleeping more now should result in a longer life because sleeping is good for us. But this simply isn’t borne out by the evidence. Although chronic undersleeping is associated with a greater risk of dying, chronic oversleeping is too. In fact, according to one meta-analysis of 16 studies on sleep duration and mortality, short sleepers had a greater risk of dying than those who slept for 7-8 hours a night, but long sleepers had an even greater risk of dying than short sleepers did.

Although this could be the result of other factors — for example, maybe the long sleepers were likelier to suffer from depression, which can also shorten lifespans — at the very least it seems unlikely, in light of the current evidence, that those who sleep more than they need to in their youth are simply "saving up" years of life for later. Although sleep requirements vary from person to person, for most healthy adults, getting 7-8 hours of sleep appears to be your best bet for a long life.

How bad is dreaming less, really?

A second possible objection is that at least some of our sleep time is spent dreaming, and that this time is more valuable than time spent in a dreamless coma. But what percentage of our sleep time do we spend dreaming? This question is difficult to answer. We report more frequent and more vivid cognitive activity during REM sleep compared with NREM sleep, and as adults we spend around 25 percent of our sleep time in REM sleep.

So let’s assume that around a third of our total sleep time is taken up with significant cognitive activities like dreams. How much do we value this time compared with the time that we spend awake?

To answer this, try to consider how many days spent in episodic dreams you’d be willing to trade in for one day of being awake and conscious. I’d be willing to trade many days of episodic dreams for a single day awake, but let’s assume that you’d only be willing to trade in two dream days for one day awake. So one hour of sleep means 20 minutes of dreams, which are worth the same as 10 minutes awake. This means that taking an hour of unnecessary sleep is akin to losing roughly 50 minutes of life.

What if you need more sleep than most people?

But what about people who biologically require more than 7-8 hours of sleep each night to avoid the effects of sleep deprivation? If my view is correct, then such people could have far lower true life expectancies than the general population.

For example, suppose that Jim and Anne will both live until they are 75, but Jim requires 11 hours of sleep each night, while Anne requires only 8 hours of sleep each night. We can work out that Anne has a waking life expectancy of 50 years, while Jim only has a waking life expectancy of just over 40 years: almost a ten year difference. Even once we correct for the value of dreams, that’s comparable with the waking life expectancy costs associated with lifelong smoking and morbid obesity.

This raises a question regarding health policy: shouldn’t we spend the same resources trying to cure Jim of his greater sleep requirements that we would spend trying to cure someone of an illness that will cause them to die eight years prematurely, all else being equal? I believe we should.

Sleep less, live more life

We generally believe that it’s a good idea to go to the gym and eat healthy food and avoid cigarettes for the sake of our longevity. But unnecessary sleep may also cut years from our life. So perhaps we should dedicate time to working out how much sleep we require and see unnecessary sleep in the same light that we see unhealthy foods or a day spent in front of the TV: as something that it’s okay to indulge in once in a while, but that we shouldn’t make a habit of.

Amanda MacAskill is a PhD student in philosophy at New York University, specializing in epistemology.

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