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 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 likely to be extremely common - as anyone who has made a long train journey will know,