The sopite syndrome is a poorly understood response to motion. Drowsiness and mood changes are the primary characteristics of the syndrome. The sopite syndrome can exist in isolation from more apparent symptoms such as nausea, can last long; after nausea has subsided, and can debilitate some individuals. It is most likely a distinct syndrome from “regular” motion sickness or common fatigue, and is of potential concern in a variety of situations. The syndrome may be particularly hazardous in transportation settings where other performance challenges (e.g., sleep deprivation) are already present.

Source : Acta Astronautica

The exact mechanism which causes sopite syndrome is still unknown, though it seems likely that it's associated with sensory mismatches between the visual, vestibular, and somatosensory systems.

In its mild form, it's evidently extremely common - as anyone who has made a long train journey will know.

Its effects are not confined to motion in vehicles etc. A recent (2019) study found evidence of the effect induced by very small movements in tall buildings due to wind. See : The effects of motion sickness and sopite syndrome on office workers in an 18-month field study of tall buildings, Journal of Wind Engineering and Industrial Aerodynamics, Volume 186, Pages 105-122.

Also see: Motion sicknessplugin-autotooltip__plain plugin-autotooltip_bigMotion sickness

Dizziness and nausea is frequently caused by unusual physical motion (or perceived motion) - e.g. air sickness, sea sickness, car sickness, VR simulation sickness etc etc.

There are three commonly-documented versions :

* Caused by motion that is felt but not seen and Infant Rockingplugin-autotooltip__plain plugin-autotooltip_bigInfant Rocking

"People tend to fall asleep when gently rocked or vibrated. Experimental studies have shown that rocking promotes sleep in humans and mice. However, the mechanisms underlying the phenomenon are not well understood."

Source : Cell Reports[Open Access]vol. 33, issue 9.