Seasickness, or motion sickness in general, has to do with how your brain perceives movement. It usually means that the brain gets conflicting messages about the body's motion or position. For instance, if you're in an interior cabin of a ship without a window, your balance-sensing (vestibular) system will feel the motion of the boat without being able to see that you're moving up and down.
The vestibular system allows us to stand upright, maintain balance and move around without falling over. It organizes the information sent from the eyes and inner ear and sends it to the central nervous system by way of a cranial nerve. When this information contradicts your body's prior experience, you get motion sickness. Take the interior cabin example. Your brain has no prior experience with feeling a severe up and down bobbing movement without being able to verify it with a look out the window. This confuses the brain and you get motion sickness. If you've never been on a boat at all, then you have no prior experience, so you may be in trouble with or without a window.
The inner ear plays a big part in the vestibular system. The outer portion of the inner ear is called the cochlea. It basically functions as a loudspeaker. It converts sound pressure impulses from the outer ear into electrical impulses and passes them on to the brain via the auditory nerve. Further inside the ear are the semicircular canals. These are the body's balance organs. They use hair cells to detect movements of the fluid within the canals. The canals are connected to auditory nerves that send these signals to the brain. If you bend over to tie your shoe, the fluid in your semicircular canal sloshes around and tells the brain, "I'm bent over now, so don't get dizzy."
Stress can also play a part in motion sickness. If you've had a bad experience with it before, you can bring on the condition through mere anticipation. Some people who regularly experience motion sickness from cars, planes and boats can induce it just by looking at the offending mode of transport.
Feeling the up and down and side-to-side motion of a boat or ship doesn't mean that you're seasick. If you feel great, then it means you have your "sea legs." If you get seasick, you'll experience some or all of the following symptoms:
How do you fight seasickness? Find out on the following page.