Lack of sleep may worsen chronic pain

We all feel the benefits of a decent night’s sleep, but could not getting enough kip be making life more painful? New research from a team at Boston Children’s Hospital and Beth Israel Deaconess Medical Centre has shed light on the link between lack of sleep and sensitivity to pain. But What might this mean for the millions of people in the UK and around the world that suffer from chronic pain? Surprisingly, it seems that a good old cup of coffee might help! Katie Haylor spoke to pain scientist and study co-author Alban Latremoliere...

Alban - We wanted to understand the possible relationship between lack of sleep and pain sensitivity. To measure that using human subjects is extremely complicated so we used animal models for which we have very little genetic variation and we can control the environments. That way we could precisely assess both their sleep/wake patterns and their pain sensitivity. We then exposed those animals to different sleep disturbances chronically to see how much it affected their pain level.

Katie - So this is the equivalent of a human having a late night, maybe going out a bit too much or watching too much telly, something like that?

Alban - That was the goal we had in mind. The mouse would make the same mistake we’re doing.

Katie - Presumably mice don’t watch box sets; how do you entertain a mouse?

Alban - It turns out that mice love chewing. I guess it comes with the fact they are a rodent and actually that makes them happy. Another thing we found mice love is cotton and so we tried to come up with strategies where mice would have to be curious and chew on some material to reach some nice cotton ball inside, and that would motivate them to stay awake. They can spend 30 minutes just having fun with them so it was actually very cute to watch.

Katie - Just at the point at which they’re wanting to go to sleep you’re going - hey, play with this cool thing and that’s the way that you chronically deprive them of sleep?

Alban - Exactly.

Katie - And you want to see if there is a link between how sleepy they are and their pain threshold. So how did you test pain sensitivity?

Alban - The starting one we used is exposing a mouse to a surface that is rather hot and then we would measure the time before they go away. The more sleepy a mouse is the sooner they are going to try and escape from a noxious stimulus so they basically don’t tolerate pain nearly as much as when they’re fully alert.

Interestingly, that’s specific to pain because if you expose them to other stimuli that are not painful, we observed either nothing or the mice, because they get sleepy, they react less.

Katie - Do we know that this pain is caused by lack of sleep? Can we say that it’s more than a correlation?

Alban - If it’s a correlation it’s a very tight one and it’s very specific. So it doesn’t prove the causality but it strongly suggests causality.

Katie - When I’m in pain I tend to take a painkiller. Did you give them painkillers - what happened there?

Alban - We tried to give them probably the same painkiller you’d take when you’re in pain and, to our surprise, that just didn’t do anything. We also tried morphine and we found that morphine’s efficacy was strongly reduced in sleep-deprived animals. The only drugs that we found that could prevent and reverse temporally this increased pain due to sleep loss were caffeine and modafinil. They are two distinct drugs that do not work on the same receptors but they both promote wakefulness and we found that these drugs were capable of restoring normal pain sensitivity in mice. Both are indirectly increasing dopamine levels in the brain, and some studies have recently shown that dopamine disruption can increase pain sensitivity and pain perception. When you don’t get enough sleep you might disrupt your dopamine transmission and that could increase your pain sensitivity. I should add though, it’s very important, that they didn’t erase sleep debt, so they are a good transient solution but your lack of sleep is still there. You cannot replace your sleep by caffeine.

Katie - What does this mean for humans?

Alban - What we found is that the type of pain you can feel when you’re not getting enough sleep is different from another source: inflammatory pain or if you cut yourself. Those are different types of pain and they can accumulate. So, for people who have chronic pain, it’s likely that the pain itself is going to prevent them getting good sleep. And the bad news is that this lack of sleep is going to cause some new pain and they are going to become even more sensitive to pain. Their treatments are either not working against this new type of pain or loose some of their efficacy overall. So, treating the pain is one thing but, maybe, to also try to help them get back their sleep, and that could help break this vicious circle and help them reduce some of the pain, and also then be more responsive to their regular painkillers.