Mostly this is caused by excess use of alcohol, as a side effect of some sedatives, Parkinson’s disease, shy-Drager syndrome, a degenerative disease characterized by a drop
in blood pressure when standing, and
dementia related to age or disease. This is something that can be diagnosed by a combination of recording BP in several positions, MRI of the brain and sleep studies. Medically this is treated with Clonazepam, however please do not start this without consulting a doctor.