Motion sickness is strongly connected with a mismatch between the senses your brain uses to judge movement.
Your inner ear may report that the body is turning, accelerating, or bouncing.
Your eyes may be looking at something that appears completely still.
Now the brain has two reports that do not agree.
Your inner ear is part of your balance system
Deep inside each ear is a set of structures that detect head movement and orientation.
Fluid shifts when your head moves. Sensory cells respond to that motion and send information to the brain.
This system works with vision and other senses to help you know where your body is and how it is moving.
Why does reading in a car cause trouble?
Imagine sitting in the back seat reading a book.
Your inner ear feels the car accelerate, brake, turn, and bounce.
But your eyes are focused on a page that stays nearly motionless relative to your body.
One system says, “We are moving.”
The other says, “This page is sitting right here.”
For people who are sensitive to that conflict, the result can be nausea, sweating, dizziness, headache, or vomiting.
Why does looking out the window help?
Looking at the horizon or distant scenery gives the visual system movement information that better matches what the inner ear is reporting.
Now both systems can agree that the body is traveling through space.
That does not cure every case, but it explains why staring at a phone in a moving vehicle is often worse than looking outside.
Why do boats cause motion sickness?
On a boat, your inner ear can detect rolling and pitching even when the cabin around you looks relatively stationary.
Going outside and looking at the horizon can again give the brain a visual reference that matches the movement.
The same basic sensory disagreement can occur in airplanes, amusement rides, virtual reality, simulators, and other situations.
Why do some people get motion sick more easily?
People vary greatly in sensitivity.
Age, genetics, migraine history, the type of motion, visibility, anxiety, fatigue, and other factors can influence the response.
Some people adapt after repeated exposure. Others remain sensitive.
Why does a movement problem make you nauseated?
The exact biological reason the sensory conflict triggers nausea is still not reduced to one simple explanation.
What is clear is that balance and autonomic systems are closely connected. When the brain detects a strong mismatch, responses involving sweating, stomach discomfort, and vomiting can follow.
So the car is not directly attacking your stomach.
Your brain is trying to reconcile two incompatible movement reports.
The digestive system somehow got copied on the email.
The same body systems connect with Why do we get brain freeze?, another everyday effect with its own underlying mechanism.
And that’s the short explanation.
Related explanations: Why do we get brain freeze?, Why do your ears sometimes ring?, and Why does your recorded voice sound weird?