Why do I get dizzy when I stand up too fast?

The medical term is orthostatic hypotension, and the mechanism is straightforward: when you stand up, gravity pulls roughly 500 to 800 milliliters of blood into the veins of your legs and abdomen. Pressure sensors in your carotid arteries normally detect this shift within seconds and tell your heart to beat faster and your blood vessels to constrict, pushing blood back upward. But sometimes that response lags — by even a second or two — and your brain gets less blood than it needs for a moment. That moment is the dizzy spell.

It’s common. Dehydration makes it worse because there’s less total blood volume to work with. So does sitting still for a long time, getting out of a hot bath, or standing up after a big meal (blood diverts to the digestive system). Aging also slows the baroreceptor reflex, which is why orthostatic hypotension becomes more frequent after about age 65.

For occasional lightheadedness, the fix is usually simple: stand up in two stages (sit up first, pause, then stand), drink enough water, and avoid crossing your legs for long periods, which can trap blood in the lower body. If it happens regularly or causes you to actually faint, see a doctor — it can sometimes point to an underlying condition like anemia, thyroid issues, or a medication side effect.