The conflict between the brain's expectation of Earth's gravity and the reality of no gravity causes space motion sickness.

Astronauts can get motion sick while splashing back down to Earth – virtual reality headsets could help them stay sharp

Source: space
Source Link: https://www.space.com/space-exploration/human-spaceflight/astronauts-can-get-motion-sick-while-splashing-back-down-to-earth-virtual-reality-headsets-could-help-them-stay-sharp