Orthostatic hypotension is a sudden fall in blood pressure that occurs when a person assumes a standing position. It may be caused by hypovolemia, a decreased amount of blood in the body, resulting from the excessive use of diuretics, vasodilators, or other types of drugs; dehydration; or prolonged bed rest. The disorder may be associated with Addison's disease, atherosclerosis (build-up of fatty deposits in the arteries), diabetes, and certain neurological disorders including Shy-Drager syndrome and other dysautonomias. The symptoms that generally occur after sudden standing include dizziness, lightheadedness, blurred vision, and syncope (temporary loss of consciousness).
This answer from the National Institute of Neurological Disorders and Stroke has been reviewed and/or edited by Dr. William D. Knopf.