You lie down to get some sleep after a long night of drinking, and the room seems to be spinning uncontrollably. What gives?
The spins happen because of an odd effect alcohol has on your ears -- specifically, on three tiny, fluid-filled structures called the semicircular canals. Inside each of these canals is a fluid called endolymph and a gelationous structure called the cupula, which is filled with cells covered in fine, hair-like stereocilia.
As you move around, the movement of the endolymph lags behind the more solid cupula, distorting and bending it -- and those little hairs. When the hairs bend, the electrical signal they send to your brain is altered, helping you to make sense of the rotations your head experiences on each of the three planes the canals sit on -- movements up and down, left and right and backward and forward -- and keep your balance.
Booze throws this system out of whack. Alcohol thins the blood, and when boozy blood travels to the inner ear, it creates a density difference between the cupula and the fluid in the canals, and distorts the cupula's shape. The little hairs bend and send a signal to your brain that tells it you’re rotating when you’re really not, and this illusion of motion makes it seem like the room is spinning.
Some of the things that you most want to do when you’re good and drunk, like lie down and close your eyes, make the sensation worse, since you don’t have any visual or physical cues to counteract the false sense of motion. Looking at a fixed object and keeping your feet planted on the ground can help lessen the effect, but there's no real way to stop it.