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Anyone who suffers from pain, whether due to an acute injury like a broken ankle or a chronic condition like back pain, knows that it can be hard to get a good night’s sleep when you are hurting. People in pain also report that when they don’t sleep well, they tend to hurt more. But why exactly? This connection between sleep and pain has been known but not understood. Now a new study reveals just how it is that sleep deprivation changes the circuitry in the brain in ways that amplify pain. And as part of the same research, an examination of the effects of natural day-to-day and night-to-night variations in sleep and pain suggests that sleep might be a useful part of therapies targeting pain.
“There’s been this intuition that there’s this vicious, looping feedback between poor sleep and increased pain,” says the lead author of the new paper, Adam Krause, a PhD student in the University of California, Berkeley sleep lab of Matthew Walker, author of the best-selling book Why We Sleep. “Here we’re providing direct evidence, giving this firm brain basis that sleep deprivation seems to directly affect the brain to increase pain. I hope that helps in treatment and, in some sense, believing patients.”
Krause and his colleagues conducted two very different studies as part of this research, which was recently published in The Journal of Neuroscience. The first in-lab experiment included 25 healthy young adults over two nights. They were allowed to sleep through one night but then forced to stay awake for 24 hours. In both conditions, rested and sleep-deprived, the scientists tested the participants’ threshold for pain using heat on their skin and found it was lower when they hadn’t slept.
They also put the participants in a functional magnetic resonance imaging machine (fMRI) to see how sleep deprivation changed the brain’s response to pain. “Pain is a neural process,” Krause says. Pain signals travel from the injury site via the spinal cord to the brain where they’re registered in the somatosensory cortex, the same brain region that registers touch. “The somatosensory cortex will estimate the location of that pain and its intensity and then signal that to the rest of the brain,” Krause explains. It will, for example, send a motor signal to get you to pull your hand away from a hot stove.
But there’s another, more complicated aspect of pain in the brain. Higher-level brain areas such as the insula cortex and the striatum appear to take the incoming pain signals and evaluate them. They also have the ability to recruit the brain’s natural painkilling system by triggering the release of endogenous opioids. “I think of these regions the gatekeepers at the highest level,” Krause says. “They evaluate pain, but they can also provide relief by blocking pain signals.”
In Krause’s study, sleep deprivation caused two kinds of changes in the brain. It increased reactivity in the somatosensory cortex, amplifying pain signals. And it decreased reactivity in the insula and striatum, the areas that evaluate and modulate pain naturally. “You put that together and you get this somewhat disturbing picture of the sleep-deprived brain when it’s in pain,” Krause says. “It’s letting in more pain signals, but the gatekeepers that normally evaluate this pain and can block it if needed are still asleep.”
This was surprising and intriguing to Krause and his colleagues. But they recognized that the extreme sleep deprivation they inflicted on subjects in the lab was not typical of real life. So they conducted a second, related study using an online survey of participants who suffered from different forms of pain—everything from a pulled muscle to chronic back pain and diabetic neuropathy. People reported on changes in their sleep from night to night and in any differences in their pain from day to day, a finding that was in line with the brain study. But there was something else that surprised the researchers again. “We saw that it was the quality of the sleep that made a difference for improving pain compared to the outright number of hours," Krause says. Quality of sleep versus quantity refers to the fact that if you only get six hours of sleep, while not ideal, those six hours will be more refreshing if they were not disrupted, if you got to sleep relatively quickly and if your brain cycled through all the natural stages of sleep.
This study still doesn’t explain what’s going on in the sleeping brain that protects people from increased pain the next day. That will be the topic of some of the follow-up studies Krause and Walker are planning.
But Krause finds the results heartening. Chronic pain is one of the most debilitating and costly health conditions there is. These studies suggest that paying attention to sleep hygiene—making sure the bedroom is cool and dark, avoiding caffeine, and the like—might be an effective piece (though probably not the entire solution) of a treatment regimen for pain. “Even though people in pain struggle to get 7-9 hours of sleep, we can still provide helpful sleep prescriptions that are designed to improve the quality of your sleep,” Krause says. “If we can make any bit of difference to reduce pain, we’re doing good work.”
Copyright Lydia Denworth 2019.
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