Benign Paroxysmal Positional Vertigo (BPPV): This condition is characterized by brief, intense episodes of vertigo triggered by specific head movements, including lying flat. The vertigo is usually accompanied by nystagmus and is due to the movement of otoliths in the inner ear.

Vestibular Migraine: Although not exclusively triggered by lying flat, vestibular migraines can cause vertigo that may worsen or be triggered by positional changes, including lying down.
Labyrinthitis: Inflammation of the inner ear can cause vertigo, and certain positions, including lying flat, might exacerbate symptoms due to changes in inner ear pressure.
Meniere's Disease: While primarily associated with hearing loss, tinnitus, and aural fullness, Meniere's disease can cause vertigo episodes that might be influenced by positional changes.

Posterior Circulation Stroke: Although less common, a stroke affecting the posterior circulation can present with vertigo. Given the potential for severe consequences, it's crucial to consider this diagnosis, especially if accompanied by other neurological deficits.
Multiple Sclerosis: In rare cases, multiple sclerosis can present with vertigo as an initial symptom, especially if there's involvement of the brainstem or cerebellum.
Acoustic Neuroma: A benign tumor on the vestibulocochlear nerve can cause vertigo, among other symptoms like hearing loss and tinnitus. Positional changes might exacerbate symptoms.

Superior Canal Dehiscence: A rare condition where there's an abnormal opening in the bone overlying the superior canal of the inner ear, leading to vertigo and nystagmus triggered by sound or pressure changes, including positional changes.
Perilymphatic Fistula: An abnormal connection between the inner and middle ear can cause vertigo, among other symptoms, and might be exacerbated by changes in position or pressure.