greyout

thumb|right|300px|Simulated stages of a greyout. A greyout is a transient loss of vision characterized by a perceived dimming of light and color, sometimes accompanied by a loss of peripheral vision. It is a precursor to fainting or a blackout and is caused by hypoxia (low brain oxygen level), often due to a loss of blood pressure.

Greyouts have a variety of possible causes: Shock, such as hypovolemia, even in mild form such as when drawing blood. Standing up suddenly (see orthostatic hypotension), especially if sick, hangover, or experiencing low blood pressure. Fatigue Hyperventilation, paradoxically: self-induced hypocapnia, such as in the fainting game or in shallow water blackout. Overexertion Severe episodes of coughing or sneezing Panic attack