Originally Posted by K.J.
I dont get a headache. But frequently when I stand up I find I black out, my vision turns black and I'm dizzy for about 5 seconds.
Lack of blood flow to your brain, in other words transient orthostatic hypotension. Get up slower, and if you're going to be sitting for a while tense the muscles in your legs, bounce/shake them a little or just generally move them around to get the blood circulating before you get up as it's natural for some blood to pool in the legs.
Look up orthostatic hypotension and see if the symptoms for it sound familiar to you.
I always have bad blood flow to my brain, and it's actually one of the causes for my frequent headaches.