system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do I get dizzy when I stand up suddenly after sitting or laying down for long periods of time?

I'm a guy and I'm in pretty good shape. I run long distances so it's not a fitness thing. It's happened for a long time, and it's never really been a problem. I just can't see for a few seconds after I stand up sometimes. Once I was napping on a hammock and got up and became dizzy. I didn't wanna look dazed and stupid and I was on an open porch so I just started walking despite not being able to see super well and accidentally stepped on a dog. Poor thing. Thanks for all the real responses.
gpt
It's called orthostatic hypotension. Basically, when you stand up, you have less blood returning to your heart for a second or two so your blood pressure drops. Your veins eventually constrict to compensate for this.