After sitting or sleeping, when I stand up I suddenly see everything black in front of me for only 3 to 4 seconds, then my sight returns normal, why that happens? if you know tell me especially if it's related to my health and thank you in advance.

Orthostatic hypotension — also called postural hypotension — is a form of low blood pressure that happens when you stand up from sitting or lying down. Orthostatic hypotension can make you feel dizzy or lightheaded, and maybe even cause you to faint.

Explanation:

Hope this helps!

they need it to survive

explanation:

answerdecreased blood pressure;