Why do some people feel sick if they read in a moving vehicle?

Motion sickness in general is caused when your inner ear and your eyes disagree about whether you’re moving. When you read in a car, your visual field stays still but your inner ear detects the twists and turns. This sensory conflict triggers nausea, possibly because the brain thinks you’ve eaten something toxic that’s making you hallucinate. Motion sickness occurs when the three parts of our body that sense movement the eyes, inner ear, and sensory nerves send different signals to the brain.

Here’s how these three systems sense movement and balance: Your eyes allow you to see that you’re moving, the nerves in the muscles and joints of your extremities allow you to feel that you’re moving, and the inner ear has canals with fluid that moves around and this allows the body to perceive motion. All of these send the sensory information to your brain, which then tells you that you are moving.

Motion sickness typically occurs when only one or two of those centers (usually the inner ear) sense you are moving, and the other(s) do not so there’s a mismatch in communication to the brain. Your brain gets confused because it’s getting mixed signals and sensing this abnormal movement, which results in nausea or vomiting.