Why do we get seasick?

The vestibular system in the inner ear senses rocking and rolling, but if you are below deck, your eyes report a still environment. This sensory conflict confuses the brain, which may trigger nausea and vomiting. One theory is that the brain interprets the mismatch as a sign of poisoning and responds by trying to empty the stomach. Going on deck and watching the horizon gives the eyes a stable reference that matches the inner ear, reducing symptoms. Seasickness usually improves after a day or two as the brain adapts, a process called habituation.