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I was asking myself this weird question.

When you get up or stand up too fast, sometimes, you see something like a dark veil, and you aren't able to see anything distinctly for 2 or 3 seconds, then it come back to normal.

Do you know what causes this weird phenomenon (biologically speaking), or am i the only weird person feeling this sometimes?

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If your interest is academic then you may wish to look further into orthostatic hypotension. I don't know why this causes visual disturbances however. –  Rory M Jul 23 at 13:33
    
the eyes can lose vision easily when they don't get enough oxygen or are otherwise impaired. I'm reminded of diabetic retinopathy - temporary eyesight loss happens when blood flow to the eyes are interrupted. this can be a sign that the cardiovascular system isn't as efficient as it should be! –  shigeta Jul 23 at 13:38
    
»[…] or am i the only weird person feeling this sometimes?« Nope, definitely not. –  PattaFeuFeu Jul 23 at 18:26

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up vote 11 down vote accepted

It's caused by a sudden shift in the pressure needed to circulate blood to your brain which your body fails to respond to sufficiently quickly. This results in a sudden loss of blood pressure termed Orthostatic Hypotension which, in term, results in a transitory reduction in the blood supply necessary for brain function. You experience a momentary loss of vision for the same reason that you would if someone strangles you.

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wow!! Never knew this exists. Can you add some more references? –  Devashish Das Jul 23 at 13:52
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Are other brain functions also affected when this happens? –  Shawn Jul 23 at 16:10
    
Other brain functions may indeed also be affected. Dizziness is another possible effect, as one example. –  keshlam Jul 23 at 21:20

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