system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why standing up quickly sometimes causes me to black out for a few seconds, and standing up and stretching is even worse.
gpt
I had this problem a lot as a teenager, and had to see a neurologist and cardiologist about it.  Here's the basic idea:

When you're lying or sitting down, your heart is in a "rested" state.  Your pulse slows down and your muscles relax, and your brain needs less blood to function as your nerves and body become calmer and calmer.

When you suddenly stand up, your muscles and heart start to quickly "wake up" and work harder, and your brain all of a sudden needs a lot more blood to send messages throughout your nervous system.

You black out briefly because your brain isn't getting enough blood from your heart in the short period of time in which it needs it.  This is usually because the message isn't getting to your heart fast enough via your nervous system, and conversely, the blood isn't reaching your brain by the time you're on your feet.

Eat more salt, drink more water, or Gatorade.  Electrolytes help your nerves work more efficiently.