I feel motion sickness every night around the same time. why ?

Most common cause : Of vertigo is benign positional vertigo. Recurrent, intense and brief episode of vertigo with a rapid change in head position. Diabetes can cause autonomic neuropathy that is associated with vertigo like instability after standing from sitting or lying down position. Pregnancy another possibility. Vestibular neuronitis if associated after viral syndrome like common cold. If persist see a physician.