That's a handful, so we asked Los Angeles optometrist Dr. Diana Risko (also a longtime rider) for some, uh, clarity. "The retinas are mostly designed for light situations," she explains. "Only around 15 percent of their receptors are made for the dark. Darkness makes the eye's pupil expand [dilate], and the headlights of an oncoming car basically set off all the receptors in your eye, giving you only a white flash in your vision." She adds, "Astigmatism [a curve to the cornea] is fairly common and often doesn't require correction. But it can make blurred vision more likely and can contribute to eyestrain."