Sopite syndrome is a poorly understood manifestation of motion sickness that causes drowsiness, mood changes, mental depression, and fatigue. Sopite syndrome may be the cause of motion-induced sleepiness that is experienced in situations such as driving, flying and being on a boat. It is also believed to be why babies fall asleep when rocked. Sopite syndrome has been linked to the vestibular system which may modulate the effects of motion that both causes ("rocking") and disturbs ("shaking awake") sleep. Astronauts experiencing sopite syndrome have dubbed it "mental viscosity" or "the space stupids.

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