People get carsick or seasick (and motion sickness in general) because their brain receives conflicting messages from their sensory systems—specifically, their inner ear (vestibular system) and their eyes (visual system).

1. Inner Ear (Vestibular System): Located in your inner ear are structures called otolith organs and semicircular canals that monitor your head position and movement. They send signals to your brain about your body’s motion, acceleration, and orientation.
2. Visual System: Your eyes provide visual cues about your surroundings and whether you’re stationary or moving. If you’re in a car or on a boat, your eyes might tell you that you’re sitting still since the inside of the vehicle or cabin appears stable.
3. Conflicting Messages: Motion sickness occurs when there’s a mismatch between what your inner ear senses (motion) and what your eyes see (stationary or different motion patterns). For example, if you’re reading a book in a moving car, your eyes register a fixed focus while your inner ear detects the swaying or jolting movements of the vehicle.
4. Brain Interpretation: Your brain struggles to reconcile these conflicting inputs and may interpret them as a potentially harmful situation, perhaps interpreting the disorientation as a sign of poisoning or toxin ingestion. As a protective response, it triggers a series of symptoms, including nausea, dizziness, and vomiting.
5. Genetics and Experience: Individual susceptibility to motion sickness varies greatly. Some people are more prone to it due to genetic factors or lack of experience with motion (e.g., children and those who haven’t spent much time on boats or in vehicles).

To cope with motion sickness, the body tries to adapt by adjusting to the motion, but in some cases, medications or behavioral techniques (like focusing on a stable horizon line) can help alleviate symptoms.