Rice's Science Journal says we experience high G-force in our everyday life, including such actions as sneezing. Grayouts or blackouts on roller coasters are usually caused by not having enough to eat or being dehydrated. It can also be caused by hypoxia or low blood oxygen heat stress, fatigue and consecutive rides.

Experiencing a "greyout" or feeling like you are about to pass out on a high-intensity roller coaster is a relatively common physiological response to high G-forces (gravitational forces), but it is not necessarily "normal" in terms of ideal health. When a coaster enters a tight turn or a steep pull-out at high speeds, positive G-forces can pull blood away from your head and toward your lower extremities. This temporary reduction in blood flow to the brain and retina can cause vision to dim (greyout) or even result in a brief loss of consciousness (G-LOC). While many enthusiasts consider this part of the thrill, it can be exacerbated by dehydration, low blood sugar, or lack of sleep. To prevent this, riders are often encouraged to stay well-hydrated and practice the "anti-G straining maneuver"—tensing leg and abdominal muscles to keep blood in the upper body. If you frequently feel faint on rides, it is wise to consult a doctor to rule out underlying blood pressure or heart issues, as the rapid physiological stress of modern "hyper-coasters" can be taxing on the cardiovascular system.