Why do I feel dizzy when I stand up fast?

When you stand up quickly, gravity causes blood to pool in your legs and lower body, temporarily reducing blood flow to your brain. Your body normally compensates for this automatically by increasing heart rate and constricting blood vessels, but there's a brief lag before these adjustments kick in. That momentary drop in blood pressure to the brain causes the lightheadedness or dizziness you feel. This is called orthostatic hypotension and it's usually harmless. It's more common when you're dehydrated, tired, or have been sitting or lying down for a long time. If it happens frequently or severely, it could signal an underlying issue like dehydration, anemia, or a cardiovascular problem worth discussing with a doctor.