Why do we twitch in our sleep?

Ever wondered what your pets are dreaming about when they suddenly kick their paws in their sleep? Maybe they’re chasing a mouse, or a cat. Maybe they’re pawing at you for treats. Or, maybe they’re just running around the house, rummaging through the garbage, scratching the couch, jumping on beds — all the things they’re not allowed to do when they’re awake. It’s hard to say. But one thing is for sure: People have been connecting these sudden twitches to dreams in animals, and in humans, for years. In humans, during the deepest stage of sleep, we have twitches in our limbs but also in our eyes. These are called rapid eye movements, REM for short. And, the science here is pretty certain. REM sleep is when we are likely dreaming. In animals, scientists also believe their twitching limbs is a likely sign of them dreaming as well. “I mean, we know we have dreams. We know that we are moving around in our dreams to some extent,” says Mark Blumberg, a neuroscientist at the University of Iowa. “So it just makes sense to think, ‘Oh, movements. Why wouldn’t they be connected?’” But then, he observed twitching in really young animals and asked, “A newborn animal has had very little waking risk experiences. What the hell are they dreaming about?” If twitching was really related to dreaming, you’d expect that the older you get and the more experiences you have, the more you’d dream, and the more you’d twitch. So, to get to the bottom of this mystery, Blumberg began experimenting on newborn rats. In a study, he surgically disconnected the part of the brain responsible for creating dreams. “We found no effect at all on twitches. And so I was like, ‘Okay, what is this about?’” If dreams were responsible for twitching, why did cutting the part of the brain responsible for them have no effect? Blumberg spoke with Unexplainable host Noam Hassenfeld about how this seemingly small question — why do we twitch in our sleep? — has fundamentally shifted how we understand the relationship between the brain and the body. Below is an excerpt of their conversation, edited for length and clarity. There’s much more in the full podcast, so listen to Unexplainable wherever you get your podcasts, including Apple Podcasts, Pandora, and Spotify. All right, Mark, just to make sure before we dive in here, when I think of sleep twitches, I think of those twitches I get, like, right when I’m falling asleep. Hypnic jerks, yeah. Is that part of this? Is that different? It’s a separate phenomenon. It’s more akin to what’s called a startle than a twitch. You’re not in REM sleep when that happens, and there are a lot of theories about it, but the fact is it’s an extremely hard thing to study. Well, if we’re just talking about these REM sleep twitches, then how common are they? Do all kinds of animals and people twitch? I’ve got a website that collects all these different videos, and what you see across different animals is that the parts of the body that the animals really, really rely on for bringing sensory information into their brain are the parts that twitch the most. So for us, you know, rapid eye movements are twitches of the eyes. We also twitch our fingers a lot when we’re adults. With cats, you see their paws moving a lot. Ferrets, you see whisker twitches; rats, you see lots of whisker twitches. They use their whiskers to learn about the world just as well as we use our eyes. And if all of these twitches aren’t just, you know, enacting dreams, how do you start figuring out what they actually are? Well, you know, the first thing you have to do is try to figure out what parts of the brain are producing this. I mean, how is this all happening? And what we started to see when we were recording brain activity is that the brains of neonates, baby rats, were much more active during sleep and much more active when animals were twitching than when they were awake, It’s one thing to think that sleep has brain activity associated with it. That was a huge finding 80 years ago. It’s another thing entirely to see that the brain activity is greater. And I mean much greater during REM sleep than during waking. And I assume it’s reasonable to think that all of that brain activity is connected with these twitches, right? Is there a way to actually test it? Yeah, I mean, the biggest problem was methodological. How do you record brain activity in a very, very small baby rat, which was the best animal for doing this sort of work. You have to figure out how to get them in a stable situation so you can drop these very fine electrodes into the brain. And so it took years to get the methods going. But what we started to see is that every time the animal twitches, 10 milliseconds later, the part of the brain that’s responsive to sensory input for that limb shows a huge burst of activity. So twitch, activity. Twitch, activity. Not the other way around. This is a sensory signal, right? So this timing here matters. If you have a twitch, and then you get a burst of activity in the brain after that twitch, then you have a pretty good idea that that’s a sensory signal that you’re picking up on. Yeah, so like a signal the brain is getting from a nerve or a muscle or something? Yeah, the sensory input. So every time you move a limb, you have sensors in your muscle, you have sensors in your skin and your joints. And those sensors, when you have movement, they produce neural signals that flow up into the brain. That’s how we know when our arms are moving or when you touch something. We have sensors all throughout our limbs. And so when the limbs were moving, that’s when we were seeing the brain activity in parts of the brain that are responsive to those types of sensory signals. Wow. Okay, so you’re essentially flipping the traditional hypothesis on its head, right? It’s not dreams causing twitches, it’s twitches causing dreams, or it’s twitches causing some impact in the brain? Yeah, I mean, obviously twitches are not going to be the sole source of all things in the dreaming brain, but that it is at least providing sensory input to the brain during sleep, that we know for a fact. So it does flip it on its head and it completely changes the calculus of what’s happening in a dreaming brain. So then why would the twitching be happening to begin with? Like, what’s the point of all of this twitching? Well, so this is where you have to start to think about what is it that’s special about twitches, right? The first thing that you notice is that the movements are discrete. And it turns out that discreteness is incredibly important. So imagine that you’re standing at a switchboard with hundreds of different switches. Let’s just say they’re neurons, and then all the wires from all of those switches lead to a whole bunch of lights. So every switch controls a different light, okay? And let’s say that those lights are muscles. If you’re sitting at that switchboard and you want to figure out which switches control which lights, you don’t just start throwing all the switches simultaneously, right? Because if you did, you’re gaining no information. All you’re seeing is a bunch of lights turn on and you’ve thrown a bunch of switches. The answer is you throw one switch at a time, you see which light comes on, and then you make that connection. And so that’s the difference between wake movements and twitches. You know, I’m sitting here talking to you, and I’m gesturing, and I’m moving all my limbs simultaneously, my posture, my neck, my eyes, everything’s moving simultaneously, right? That’s waking. One of the characteristics of waking movements is that they’re continuous, and they’re simultaneous, and they’re highly complex. But when you’re twitching, one twitch at a time, you ping your body. And the body pings you back. And then you know that the first thing is related to the second thing and that’s the discreteness of twitching. And that explains why these animals are twitching so much. You never grow and develop more than you do when you are young. So, your theory is that the power goes out and they’re flipping switches in a fuse box to see which switch controls which light because there’s no other stimuli coming in, right? They’re in a controlled environment, so they’re essentially doing sort of an experiment to learn their own body? Yeah, exactly. It’s like they’re bootstrapping their system. They’re self-organizing their sensory motor system and it’s done from within. It’s a big mystery as to how we develop things like our sensory motor system. How do you actually learn about your body when you’re a newborn rat or a human and you’re born, you have no idea how your body is formed. You have no idea how it moves, and it’s going to be changing every single day as you grow and figure out new things, right? So how do you figure out how to move that body in real time through the process of development? You can’t prescribe this. You can’t blueprint this. There’s no genetic mechanism that can tell you exactly how you’re going to be on day three versus day five. So you need to have a system that’s highly adaptable. And if twitching is about learning, we would assume younger animals would twitch more, is that the case? Absolutely. And then, I mean, older animals also twitch. Yes. Why would they be twitching? Good question. First, we don’t twitch as much when we’re older. But second, some animals do twitch quite a lot. And the part of the body that twitches matters. And this is just a theory because nobody has really explored it with the level of sophistication that we need. But we have to calibrate our systems, you know, over the day we get tired, we lose control, you know, our vision gets worse and worse through the day. And then you wake up the next day and you’re rejuvenated. I think it’s possible that twitches continue throughout life, for some parts of the body, for that purpose; to calibrate a weary system. And there’s some hints out there in the world that this could be happening, including work that was done in humans. But they’re mostly hints and it needs to be done more systematically. And why do you think the scientific community missed this for so long, missed understanding twitches as a developmental process? Because when you label something as a byproduct of dreams, why would anybody spend their time studying it? Like, it’s just closing off further inquiry? Yeah, I mean, I don’t want to be too flippant about it. Dreams are fascinating, but they’re kind of a red herring when it comes to studying sleep. There is, to my mind, many, many fascinating things about sleep that have nothing to do with dreams, and the focus on dreams is kind of a distraction from what really matters.

What really matters

Why do we twitch in our sleep?