This is called postural or orthostatic hypotension. It’s experienced by a lot of the population, especially those who already have low blood pressure. It can be a sign of anemia, especially if you’re female. Basically what happens is that after you sit for a long time, your blood vessels are constricted and your blood pools in your legs, so when you finally stand up, those blood vessels don’t bring blood back to your heart fast enough for you blood pressure to stay constant. It’s like when you walk down the stairs and you don’t remember that there’s one more step, but instead of you, it’s just your brain having a temporary oxygen deprivation. It can be scary and really uncomfortable, but it’s pretty normal – you just have to get used to standing up a little more slowly. If the head rush feeling or the blackout lasts longer than a minute or two, or if you totally lose consciousness, you should see a doctor, and if it’s happening fairly frequently, you might want to get checked for anemia or some other cause. I’m not a doctor, so don’t take this as a diagnosis, but it’s probably just postural hypotension. However, my aunt thought she just had low blood pressure, and it turns out she had a blood clot in her brain and and an oversized heart. If you’re worried, see a doctor. And bear in mind that if you experience light-headedness frequently, you’re at a greater risk for concussion, which in turn puts you at a greater risk for blood clots in your brain and seizures.