What the forget? Why is everyone saying "it's normal, it happens to everybody" I've never experienced this once, and I've stayed on my computer for 8+ hours at a time.
You need to be laying down, thats the most effective. Also standing up very quickly is another then. Laydown and play games for like 4 hours and suddenly stand up. Make sure theres no sharp objects nearby
Contrary to what everyone is saying, it is not blood rushing in or out of the brain.
White flashes and impaired hearing is NOT common with the typical "quickly standing up/sitting down".
Go see a doctor.  It's either a cardiac problem or a neurological problem.
Its normal
When there is no blood to a specific part of the body, it becomes "disabled" to prevent damage without oxygen as much as possible (Its like the cells are holding their breath, while they are holding it they stop whatever they are doing)
Thats why when you stand up quickly, the blood is flushed from your head thanks to gravity and for your body to prevent permanent damage your eyes, ears, and even your smelling, it all gets disabled to prevent damage.