Lying sideways dizziness is mainly caused by positional vertigo, which can be seen in benign paroxysmal positional vertigo and vertebral artery cervical spondylosis. 1. Benign paroxysmal positional vertigo: it is mainly caused by otolith dislodgement, which can be induced by various reasons, such as head trauma, vestibular neuritis, otosclerosis, etc. The otolith dislodgement of the posterior semicircular canal is the most common. When the head position changes while lying down, the dislodged otolith fragments can be displaced to the equilibrium structures such as the semicircular canals and the crest cap of the jugular ridge, thus inducing vertigo. 2. Vertebral artery-type cervical spondylosis: This disease is related to degenerative changes of the cervical spine. Poor sleeping posture and change of head position may lead to vertebral artery compression, thus causing insufficient blood supply to the brain and vertigo and other symptoms. Other reasons can also lead to side lying and other positional changes in the symptoms of dizziness, dizziness symptoms if the long time does not relieve or accompanied by other discomforts, should consult a doctor in a timely manner, so as not to delay treatment.