Actually, your balance relies on a combination of fluid balance on the inner ear which it compares to the visual image of the horizon when you are looking at it. In meditation, when you close your eyes, it has to rely on the fluid 'horizon' to tell you which way is up. But in meditation, as you disengage from your senses, the brain then relies almost completely on the inner ear and other peripheral senses- so if there is any problem with the fluid, such as inflammation, or other structural (even temporary) change, then you will find yourself thinking you're up but actually being sideways, and when you open your eyes- BAM! you're jolted back to up (or down.) This is very disconcerting if you blow out your eardrum (as I did when scuba diving many years ago.) While healing I couldn't tell straight up from sideways even with my eyes open. A bar stool was out of the question- my bf had to hold me up, and it was very funny for all those around me.
(edited due to a changed belief about hypnagogics).