What causes motion sickness?

Your sense of balance draws on three inputs: the vestibular system in the inner ear detecting acceleration and orientation, vision, and proprioception from muscles and joints. Motion sickness arises when those inputs disagree. Reading in a moving vehicle is the clearest case, since your inner ear reports motion while your eyes report a fixed page. Below deck on a ship, the cabin appears stationary while your body feels the swell. In virtual reality the opposite occurs, with the eyes reporting motion the body does not feel, which is why VR sickness affects people who never get carsick.

Why sensory conflict produces nausea specifically is not fully settled. The most cited hypothesis proposes that the brain interprets the mismatch as a symptom of neurotoxin ingestion, since poisons can disrupt sensory integration, and responds with the defense evolved for that situation, which is vomiting. The explanation is plausible and unproven, but it accounts for the otherwise strange pairing of a balance problem with a digestive response.

What actually helps. Look at the horizon or a fixed distant point, which restores agreement between vision and the inner ear. Sit where motion is least, meaning the front seat of a car, over the wing on a plane, or midship on a lower deck. Drive if you can, since drivers rarely get sick because they anticipate the motion. Get fresh air, avoid reading and screens, and eat lightly beforehand rather than not at all. Antihistamines such as dimenhydrinate and meclizine work but cause drowsiness, and scopolamine patches are effective for longer trips and require a prescription. Ginger has modest supporting evidence. Acupressure bands have not performed well in controlled trials. Susceptibility varies enormously between people, tends to peak in childhood, and often decreases with age and with repeated exposure as the brain adapts.