It’s the biggest performance hack of all, and all it costs is a third of your time on this planet, give or take an hour or two. I’m talking about sleep, which has become even more of an obsession among athletes and other aspirants in recent years. Forget Thomas Edison and his four hours a night: The hallmark of a great athlete these days is “high sleepability”. This is the ability to fall asleep quickly and easily when the opportunity presents itself, even when you are not deprived of sleep.
With this noble aim in mind, I bring you a new review paper, published in this month’s edition of Sports Medicine, on the relationship between sleep and sports injuries, a subject I’ve written on a number of times. The general conclusion, based on 12 prospective studies, is: – Oh wait … apparently there is “not enough evidence” to link sleep disorders to injury in most of the populations studied. This non-finding is a bit surprising and worth digging a little deeper because it tells us about the dangers of being overly enthusiastic about seemingly obvious performance aids.
First disclaimer: I’m a huge fan of sleep. I make a fetish when I try to spend enough hours in bed that I practically never have to wake up with an alarm clock. I mention this because I suspect a lot of recent sleep boosterism has come from people like me who already tend to spend more than eight hours a night and who are willing to accept evidence to suggest they do do the right thing. Whenever I read an article about a supposed new performance enhancing addition, my antennas are on high alert for research design flaws or conflicts of interest. I’m probably less critical of something like sleep. And I’m not the only one.
In 2015, I wrote about a study in the Journal of Pediatric Orthopedics that analyzed injury data from 112 athletes at a high-end high school in Los Angeles. I’ve attached this graph, which shows an obvious relationship between the risk of injury and the number of self-reported hours of sleep per night:
(Illustration: Alex Hutchinson)
The club looks pretty clear here: athletes who got eight or more hours of sleep a night were injured much less often. But does a lack of sleep actually cause injuries? It’s harder to say.
Several different causal mechanisms are discussed in the new Review of Sports Medicine, authored by a group at Towson University under the direction of Devon Dobrosielski. Sleep deprivation has been shown to suppress testosterone and growth hormone production and increase cortisol levels, which can weaken muscles and make you more prone to injury. Drowsiness can also slow your reaction times and lead to more loss of attention, which can increase the risk of a twisted ankle or a puck on your face. But there are also many non-causal possibilities: It can simply be that athletes who adhere to the rule “Lights out at 10 p.m.” avoid risky games and sudden increases in training volume more carefully. Or a separate factor like overtraining can both disrupt sleep and increase the risk of injury.
I was particularly interested in this topic because this LA high school study made a controversial appearance in sleep scientist Matthew Walker’s 2017 bestseller Why We Sleep. He even included the same graphic in his book – with one crucial difference. As a blogger named Alexey Guzey pointed out, he skipped the bar for five hours of sleep, so it looked like the risk of injury rose steadily and inexorably with fewer hours of sleep. (Walker reportedly changed the graphics for subsequent editions of the book.)
There is an interesting discussion here about the “right” level of simplification. Effective science communication always involves cutting out extraneous details, and this cutting process is inherently subjective. One could argue that knowing what to leave out without distorting the message is the key skill in science journalism. And to be clear, I think Walker misunderstood that balance in his original graph. But I don’t think it’s necessarily because it’s in Big Sleep’s pocket or something nefarious. Instead, it seems to me to be more of an example of what I talked about above: our tendency to take positive sleep research uncritically because it seems so natural and harmless and, in a sense, morally correct: when we’re good guys and girls and on time ins Go to bed, the injury fairy will leave us alone.
But back to Dobrosielski’s report: he and his colleagues found 12 studies that met their inclusion standards. All dealt with adult athletes and all were prospective, which meant they had an initial estimate of how much or how long they slept, followed by a period of time monitoring injuries. Six of the studies found no significant association between sleep and injury; The other six did, but the studies were so diverse that there weren’t any general patterns about what types of injuries or athletes or sleep patterns were most important.
It’s worth noting that an earlier 2019 review looked at the evidence in adolescents rather than adult athletes. In that study, they concluded that adolescents who were chronically poor in sleep – a definition that varied between studies but usually meant less than eight hours a night – were 58 percent more likely to suffer from sports injuries. However, this estimate was based on only three studies and still does not clarify the difference between correlation and causality.
In the end, I continue to believe that sleep is good for us and that people who insist that they “only need” five or six hours a night are fooling themselves. But the truth is, as Canadian Olympic team sleep scientist Charles Samuels told me a few years ago, there really isn’t that much evidence to support these assumptions. In particular, the connection between sleep time and the risk of injury seems to me to be increasingly shaky due to the new assessment. In times of relentless self-optimization, I have to think of one of the other wisdom nuggets from Samuels: There are no bonus points for sleeping better than normal. Time in bed is valuable, but not a magical panacea. If you miss your bedtime every now and then, you won’t lose sleep on it.
Hat tip to Chris Yates for additional research. If you’d like to learn more about Sweat Science, visit me on Twitter and Facebook, subscribe to the email newsletter, and read my book Endure: Mind, Body, and the Strangely Elastic Limits of Human Achievement.
Main photo: JP Danko / Stocksy