Sleep-talking (or “Somniloquy”, as us fancy-pants scientist people call it) is a phenomena where a sleeping person starts talking. For example, the internet sensation The Sleep Talkin Man. Sleep talking can range from grunts or moans to relatively clear speech. While most people know what sleep talking is (there was even a hit song about it that’s older than I am) fewer people know what causes it.

To explain what happens when someone’s talking in their sleep, we first need to talk about 1) what happens during sleep and 2) what happens when we talk normally.

• Sleeping normally: One of the weirder things about sleep talking is that it happens at all. When you’re asleep normally, your muscles undergo atony during the stage of sleep called Rapid Eye Movement, or REM sleep. Basically, your muscles release and go into a state of relaxation or paralysis. If you’ve ever woken suddenly and been unable to move, it’s because your body is still in that state. This serves an important purpose: when we dream we can rehearse movements without actually moving around and hurting ourselves. Of course, the system isn’t perfect. When your muscles fail to “turn off” while you dream, you’ll end up acting out your dream and sleep walking. This is particularly problematic for people with narcolepsy.
• Speaking while awake: So speech is an incredibly complex process. Between a tenth and a third of a second before you begin to speak you start brain activation in the insula. This is where you plan the movements you’ll need to successfully speak. These come in three main stages, that I like to call breathing, vibrating and tonguing. All speech comes from breath, so you need to inhale in preparation for speaking. Normal exhalation won’t work for speaking, though–it’s too fast–so you switch on your intercostal muscles, in the walls of your ribcage, to help your lungs empty more slowly. Next, you need to tighten your vocal folds as you force air through them. This makes them vibrate (like so) and gives you the actual sound of your voice. By putting different amounts of pressure on your vocal folds you can change your pitch or the quality of your voice. Finally, your mouth needs to manipulate the buzzing sound your vocal folds make to make the specific speech sounds you need. You might flick your tongue, bring your teeth to your lips, or open your soft palate so that air goes through your nose instead of your mouth. And voila! You’re speaking.

Ok, so, it seems like sleep talking shouldn’t really happen, then. When you’re asleep your muscles are all turned off and they certainly don’t seem up to the multi-stage process that is speech production. Besides, there’s no need for us to be making speech movements anyway, right? Wrong. You actually use your speech planning processes even if you’re not planning to speak aloud. I’ve already talked about the motor theory of speech perception, which suggests that we use our speech planning mechanisms to understand speech. And it’s not just speech perception. When reading silently, we still plan out the speech movements we’d make if we were to read out loud (though the effect is smaller with more fluent readers). So you sometimes do all the planning work even if you’re not going to say anything… and one of the times you do that is when you’re asleep. Usually, your muscles are all turned off when you’re asleep. But, sometimes, especially in young children or people with PTSD, the system will occasionally stop working as well. And if it happens to stop working when you’re dreaming that you’re talking and therefore planning out your speech movements? You start sleep talking.

Of course, all of this means that some of the things that we’ve all heard about about sleep talking are actually myths. Admissions of guilt while asleep, for example, aren’t reliable and not admissible in court. (Unless, of course, you really did put that purple beaver in the banana pudding.) It’s also very common; about 50% of children talk in their sleep. Unless it’s causing problems–like waking people you’re sleeping with–sleep talking isn’t generally problematic. But you can help reduce the severity by getting enough sleep (which is probably a good goal anyway), and avoiding alcohol and drugs.