{♫Intro♫}.

It's a story that plays out every Friday night somewhere. People are having a good time with their friends when they have a few too many drinks and suddenly everything is spinning.

They might try to lay down to try to sleep it off, but that just makes it worse. Yep, they've got the spins. And, strange as it might seem, that's not because the alcohol they drank is messing with their brain.

The spins are a form of vertigo, which is the illusion that you or the world around you are moving. But that illusion doesn't happen because alcohol affects neurons. I mean, don't get me wrong, alcohol does affect neurons.

And it messes with your brain in a lot of ways—like, it lowers your inhibitions and impairs your memory. But the spins happen for a different reason— and to understand that, you first have to understand how your brain knows when you're moving. Deep within each ear are three semicircular canals — curved tubes which are oriented at different angles and filled with a fluid called endolymph.

At one end of each tube is a cupula, a jelly-like mound that has a bunch of tiny sensory receptors called hair cells embedded in it. As the name implies, these cells have thin, hair-like projections coming out of them. And when those quote “hairs” bend, the cells send electrical signals to the brain that say “hey, something is happening”.

For the hair cells in the cupula, that something is that the head is turning. When your head turns, the semicircular canals rotate along with it. But inertia makes the endolymph inside lag behind a little bit and therefore press up against the cupulas.

That pressure bends the cupulas and their hair cells. The brain figures out exactly how your head is turning based on which cupulas in which ears are sending it signals that they're being bent. This is why ballerinas use the technique of spotting when doing a bunch of pirouettes.

By keeping their head facing one direction as much as possible, they minimize the amount of time where endolymph is pressing on their cupulas, and essentially “trick” their brains into thinking they're spinning less. But alcohol messes with this whole system for one simple reason: it's not as dense as your endolymph. There aren't a ton of blood vessels in your inner ear— and most of them are in or near the cupula.

Because of that, alcohol present in your blood diffuses into the cupula faster than it diffuses into the endolymph, making the jelly-like structure less dense than its neighboring fluid. The difference in relative densities changes the pressure on the cupula, which causes it to distort. And when that distortion happens, the hair cells bend and start signaling to the brain that the head is turning — even though it's staying completely still….

And you feel like you're the ballerina doing those pirouettes. At this point, most people just want to go lie down, close their eyes, and sleep it off—but in the cruelest of ironies, that actually just makes the spins worse. See, there's supposed to be a separate system in the inner ear that detects forward-and-back motion and gravity.

But if the densities of the cupula and the endolymph aren't exactly equal, the semicircular canal system veers out of its lane. Instead of the cupula essentially floating in the endolymph, it becomes lighter than it. And that means it will distort from any change in head position.

So it'll sound the rotation alarm in response to linear movements and gravity, too. You can see evidence of this happening if someone lays down when they're drunk. Their eyes will start to drift away from the floor and then quickly move towards the side they're laying on.

This is called positional alcohol nystagmus, and it happens because the brain attempts to keep the gaze steady as the head turns —until it realizes the head isn't turning, and snaps the eyes back. So, to recap: laying down, particularly on your side, makes the spins worse because your brain interprets gravity as rotation. So, you don't want to lay down.

And you really don't want to close your eyes —they've actually been holding this whole situation in check. That's because your ears aren't the only part of you that senses if you're moving. Your brain likes to use as many sources of information as possible to figure out what's happening around you.

So it combines messages from several different sensory systems. Psychologists call this multisensory integration. And during the spins, the inner ear is sending the message of spinning, but the visual system is correctly telling your brain that everything is still.

And it just so happens that, as a species, humans rely on our sense of vision more than any other. So when there's a conflict between sensory systems, your brain tends to default to trusting your eyes. If you take away that visual input by closing your eyes, the only information your brain gets is your ears' message of spinning.

So it fully commits to that— and your spins feel even worse. There are a lot of suggestions for how to ease the spins. Staying upright and keeping your eyes open can definitely help.

Some people suggest that if you do lay down, you should keep one foot on the floor so that the brain has some extra information about which direction is down. And some people find it's helpful to do something that relies heavily on vision—like watching a movie. But whether such activities help by actively engaging the visual system and overriding the spin signals is unclear.

They might just distract you from feeling crummy. If all else fails: wait it out. After about three hours, alcohol from your blood will have diffused throughout the endolymph.

So the densities of the cupula and endolymph will have reached equilibrium and the spinning will stop. Then, you just have to worry about a hangover. Thanks for watching this episode of SciShow Psych!

We hope you enjoyed today's tour of your brain. If you want to learn more about how alcohol affects you, you might like our episode on the connection between booze and creativity. So why not watch that one next?

And please remember to always drink responsibly! {♫Outro♫}.