What does the evidence show?
Keep getting your eight hours, but the science is more complicated than you might expect
A few weeks ago, I delved into the research on post-exercise recovery and concluded that the research backing virtually all recovery techniques is very thin—with the exception of sleep and diet. But I have a confession to make: even the research on sleep is pretty sparse. Sure, it seems self-evident that a good night’s sleep is a powerful restorative. There’s not a lot of research, though, that proves it, or that gives us much information about exactly how much we need, when, and (insert eye roll) in what sort of pajamas.
So it’s interesting to see that there’s a bunch of sleep research due to be presented at the annual meeting of the American College of Sports Medicine in Minneapolis next week. Conference presentations are inherently preliminary, so their conclusions have to be taken with a grain of salt—but they offer a sneak peek at the questions that researchers in a given field are currently tackling. Here’s a look at a few of the sleep studies that caught my eye:
Does one night of bad sleep trump a month of good sleep?
A team at the University of Wisconsin-Madison, led by Andrew Watson, put 59 female teen soccer players through a battery of physical tests after having them fill out a questionnaire about their sleep the previous night and over the previous month. It’s not immediately obvious to me which should have a greater effect on your physical performance, so I was interested to see the results.
The physical tests included VO2max, which you can think of as a measure of fitness, and a time-to-exhaustion test on a stationary bicycle, which you can think of as a measure of performance. Of course, better fitness generally leads to better performance, but they’re not identical. You can have amazing fitness but still produce a crappy race if you're nervous or unmotivated—or, potentially, as this study sought to test, if you're not well-rested.
The best predictor of VO2max was the average duration of sleep over the previous month. Those who reported sleeping more than 8 hours a night had an average VO2max of 50.4 ml/kg/min, compared to 45.2 for those who slept less. That’s consistent with the idea that getting enough sleep allows you to recover between training sessions so that you experience greater improvements in fitness. (Of course, there are plenty of other ways you could spin it. Maybe the people who train hardest get fitter, and are also the most tired at night. There are no final answers here.)
Interestingly, though, the big sleepers (based on the prior month’s sleep) didn’t necessarily have the best performance in the time-to-exhaustion test. And the length of the previous night’s sleep didn’t seem to be a key factor either; those who slept more than 8 hours lasted a bit longer, but the difference wasn’t statistically significant. Instead, the key factor seemed to the ratio of acute sleep (last night’s rest) to chronic sleep (the average nightly rest in the last month). Those who had slept more than usual the previous night lasted 16.4 minutes on average, while those who had slept less than usual the night before lasted just 14.9 minutes.
Again, there are lots of possible interpretations here. The simplest is that getting good sleep on a regular basis helps you adapt to training and get fitter, while getting less sleep than you’re used to the night before a competition can hinder your performance. That sounds like bad news for those who tend to sleep poorly the night before a big competition—which, frankly, is most of us. But there may be a difference between relatively unimportant lab tests, where the concentration lapses and demotivation that follow a restless night might easily interfere with your performance, and actual real-life competitions where the stakes are high.
What happens if you miss an entire night of sleep?
If the previous study has you worried about sleeping poorly before a competition, this one should reassure you. A research team led by Edward Coyle at the University of Texas at Austin put 27 volunteers, mostly West Point cadets, through a set of physical tests before and after missing a full night of sleep. The measures included VO2max and time-to-exhaustion tests, as well as a bunch of other things like neuromuscular power, reaction time, cognitive performance, muscle oxygen extraction, and so on. The differences after staying awake for 36 consecutive hours, the researchers report, were “small or non-existent.”
This is consistent with other studies finding that your muscle function isn’t greatly affected by short-term loss of sleep. Instead, the differences seem to be mostly in your mental state. The fact that the subjects in this study performed just as well in their time-to-exhaustion test after missing a night of sleep may have something to do with the parameters of the experiment. Their test lasted less than 6 minutes on average, compared to about 15 minutes in the previous study; longer tests tend to be more susceptible to mental factors. Or it may say something about the difference between teen soccer players and West Point cadets. It’s hard to know for sure, but it suggests that a night of tossing and turning isn’t something to lose even more sleep over if you’re sufficiently motivated.
Can sleep coaching improve performance?
Shifting back to the longer-term benefits of getting good sleep on a regular basis, a study led by Eric Neufeld of the David Geffen School of Medicine at UCLA offers a more direct test of the idea that better sleep leads to better fitness gains. This study involved 38 members of a fitness club who each received 10-minute personalized education sessions from their personal trainer once a week for 12 weeks. For 22 of the subjects, the focus of these sessions was “sleep improvement,” while the rest of the subjects received more general wellness information. All the subjects were doing similar physical training four times a week.
The results were striking: the group that received sleep coaching showed significantly greater improvements in VO2max, lactate threshold, ventilatory threshold, lower body power, body composition, fat mass, blood sugar, and heart-rate variability. Basically everything they tested was better in the sleep group.
Now, you always have to be cautious when a seemingly simple intervention produces amazing results. It may be, for example, that the trainers were really enthusiastic about the sleep coaching, and unintentionally conveyed that enthusiasm to their subjects, convincing them to train harder. This is the sort of question we’ll hope to get an answer for when the study makes it into a peer-reviewed journal. To be honest, the results strike me as a little too good to be replicable—but on the other hand, they’re consistent with my own preconceived feeling that sleep is pretty much the best thing in the world.
Which brings me back to where I started. I really think that, as many sports scientists have long said, if you’re braving ice baths and downing beet juice and blowing cash on massages and compression socks, but you’re not getting 8 hours of sleep a night, you’ve got your priorities backwards. As far as the science goes, though, that’s still just an opinion—so let’s hope studies like the ones above can start to change that.
My new book, Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance, with a foreword by Malcolm Gladwell, is now available. For more, join me on Twitter and Facebook, and sign up for the Sweat Science email newsletter.