It's something we feel we've always known: if you can't sleep, you need to exercise more. Wear yourself out, make yourself good and tired, you'll sleep like a baby!
So when I started having trouble sleeping, I just figured I needed to work out more. Of course, it kind of figures that often, you have trouble sleeping because of life stress, which often means you're really busy, which in turn means it probably puts MORE stress in your life just trying to find the time to work out. But that's just details.
So sometimes, when I catch myself constantly waking up in a panic over several days, I'll fit in some hard exercise. Maybe I'll go for a long run, or try a really hard new class or something. By the time I go to bed I am WIPED. Physically and mentally. My body is so exhausted that the feeling of lying down is one of total bliss.
...so why can't I SLEEP?!?!
Turns out I was suffering under expectations that were a little too high for reality.
Glazer Baron, et al. "Exercise to Improve Sleep in Insomnia: Exploration of the Bidirectional Effects" Journal of Clinical Sleep Medicine, 2013.
(Side note: before we get into this I feel obligated to point out that I am writing this because I couldn't sleep.)
First off, we're not wrong. Exercise DOES improve sleep. It does. But not necessarily immediately. And perhaps, instead, we should ask a different question. Instead of asking how exercise impacts sleep, perhaps we should ask how sleep impacts exercise.
The authors of this study were looking at exercise and sleep, especially in the elderly. We all sleep less as we get older, but chronic insomnia is a different beast entirely. When we don't get enough sleep, we get snappish, have trouble concentrating, suffer from daytime sleepiness, and are more susceptible to things like getting sick, or getting in to accidents.
There are of course various pills you can take to help increase your sleep (remember the glowing Lunesta moth?). But they have side effects, you can become addicted to them (in the sense that you can't sleep without them and require increasing doses), and in general, people tend to prefer a more "natural" (to them) cure, such as a lifestyle change (though of course, then you have to keep it up, so).
Exercise is one of the ones that people prefer. It's got many benefits in addition to helping you sleep. But how much does it really help you to sleep?
To find this out, the authors took 11 women (small sample, which doesn't thrill me), and asked them to exercise 3 times a week at a specific intensity (75% max heartrate) for 16 weeks, but allowed them to exercise more or less, as they chose. They wore activity wristbands so that the authors could be sure they were sticking to the treatment, and also so that they could assess their sleep both before and after. They looked at how much they exercised, how well they slept, etc.
And what they found was not that exercise predicted sleep. Instead, sleep predicted exercise.
The authors found that working out did NOT immediately affect your next night's sleep, though after 16 weeks of the study, people slept about an hour more per night than they had before. But on any given night, whether you worked out didn't affect how well you slept. But instead, how much you slept the night before predicted how much exercise you got the next day. What you can see above is trajectory, how much exercise someone got as dependent on the amount of sleep they got the night before. You can see the trajectories go down. The less sleep people got, the less they worked out. This makes major sense to me, as since I did not get a lot of sleep last night, I was both too tired and unproductive to make a go at it today.
Now, you might think: well, then we kind of KNEW that, so what difference does it make? It might, actually, make some big differences in how people approach treatment of insomnia. For example, long term exercise does help people sleep more, but it requires the development of the HABIT of constant exercise. And if someone's not getting a lot of sleep the night before, they are less likely to exercise, and so less likely to get in the habit. So it might be a good idea to pair the development of exercise with another type of treatment when you start out treating insomnia.
And of course, it also puts paid to the idea that you can work yourself into a good exhausted sleep in a single day. If you're going to make it work, you've got to keep up the habit. Me, I'm off to exercise.