What causes sudden dizziness when standing up quickly?

When you stand up quickly, gravity causes blood to pool in your legs and lower body, temporarily reducing blood flow to the brain. This brief drop in blood pressure is called orthostatic hypotension. Normally, your autonomic nervous system compensates almost instantly by constricting blood vessels and increasing heart rate, but sometimes this response is too slow. Common triggers include dehydration, prolonged sitting or lying down, certain medications like blood pressure drugs or diuretics, and conditions like anemia or diabetes. Age also plays a role, as the compensatory reflex slows over time. The resulting lightheadedness or dizziness typically lasts only a few seconds. Persistent or severe episodes warrant medical evaluation, as they can indicate underlying cardiovascular or neurological issues.