Editor-In-Chief: C. Michael Gibson, M.S., M.D. 
Synonyms and keywords: greying out; brown out; grey-out
A brownout, or grey-out, is a transient loss of vision characterized by a perceived dimming of light accompanied by a brown hue and a loss of peripheral vision. It is a precursor to fainting or a blackout and is caused by hypoxia, a loss of blood pressure or restriction of blood flow to the brain. It is commonly experienced when suddenly standing up (see orthostatic hypotension), especially if sick, hungover, or suffering from low blood pressure or shock. Usually recovery is rapid and a brownout can be readily reversed by lying down.
A brownout may be brought on intentionally, although inadvisedly, through hyperventilation, the process is described in the article the choking game under self-induced hypocapnia. Hyperventilation may also bring it about unintentionally as described in the article shallow water blackout.
A brownout, or grey-out, may also be experienced by aircraft pilots pulling high positive g-forces as when pulling up into a loop or a tight turn forcing blood to the lower extremities of the body and lowering blood pressure in the brain. This is the reverse of a redout, or a reddening of the vision, which is the result of negative G forces caused by performing an outside loop, that is by pushing the nose of the aircraft down. Redouts are potentially dangerous and can cause retinal damage and hemorrhagic stroke. Pilots of high performance aircraft can increase their resistance to brownout by using a G-suit, which controls the pooling of blood in the lower limbs but there is no suit yet capable of controlling a redout. In both cases symptoms may be remedied immediately by easing pressure on the joystick. Continued, or heavy G force will rapidly progress to g-LOC (G force induced Loss of Consciousness).
Another common occurrence of brownouts are in roller coaster riders. Many roller coasters put riders through high positive G forces, particularly in vertical loops and helices. Roller coasters rarely have high enough negative G forces to induce redouts, as most low-G elements are designed to simulate weightlessness.
Also, grey-out may occur after donating blood, resulting in a loss of blood pressure and insufficient blood reaching the brain. However, the onset of such grey-outs is usually gradual and preventable by lying down so that the blood does not need to work against gravity to reach the brain.