(WHTM) — You may have seen videos of riders on a roller coaster having fun and screaming with joy, and then suddenly they pass out for a few seconds before regaining consciousness.

People can pass out, gray out, or experience loss of peripheral vision on coasters for a number of reasons. But according to the Rice Undergraduate Science Research Journal, the main reason is positive G-forces.

G-force is the measure of acceleration divided by the gravitational constant. When sitting at a desk, or at a table, you experience only one G-force, or G. That one G is the acceleration we feel due to gravity alone.

But on a roller coaster, your body can feel much heavier than it would if you are sitting down at a desk. For example, Skyrush at Hersheypark features a 200-foot drop. At the bottom of the first drop, the ride hits five Gs. That means that your body is taking on five times the force of gravity, making your body feel five times heavier than normal.

This high G-force can push heads down and have blood rush from your brain down to your feet, which in turn lowers the oxygen level in your brain, which may lead to gray outs, loss of peripheral vision (known as tunnel vision), or temporary blindness.

Don’t worry, though. Coasters are built so our bodies only deal with this G-force for seconds at a time.

The powerful G-force only lasts for a second or two, and it can only cause damage if you are exposed to it for prolonged periods of time. 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.

So before getting on a roller coaster, be sure to eat and drink enough to lower the risk of passing out, thereby increasing your chance of enjoying the ride.