A greyout (US English grayout) 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:
Usually recovery is rapid and a greyout can be readily reversed by lying down. This way, the cardiovascular system does not need to work against gravity for blood to reach the brain.
A grey-out may 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 greyout 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 flight controls. Continued, or heavy g-force will rapidly progress to g-LOC (g-force induced Loss of Consciousness).
Surprisingly, even during a heavy grey-out, where the visual system is severely impaired, pilots can still hear, feel, and speak. In other words, complete grey-out and loss of consciousness are separate events.
Another common occurrence of greyouts is in roller coaster riders. Many roller coasters put riders through positive g-forces, particularly in vertical loops and helixes. Roller coasters rarely have high enough negative g-forces to induce redouts, as most low-g elements are designed to simulate weightlessness.