Why can motion sickness occur when a person is below deck on a moving boat?

Motion sickness occurs when the brain receives contradictory sensory information regarding the body's movement and position.1

When you are below deck on a moving boat, your vestibular system—located within the inner ear—detects the physical motion of the ship, such as linear and angular accelerations caused by bobbing up and down. However, because the cabin moves in synchrony with you, your eyes register a relatively stable environment.1

This mismatch between the vestibular system's internal cues of movement and the visual system's perception of a stationary scene confuses the brain. In response, the body may trigger symptoms such as dizziness, nausea, and vomiting. This physiological reaction highlights the importance of sensory integration in movement perception; when these systems fail to align, the resulting conflict can lead to the discomfort associated with motion sickness.12