Regular HoursExcessive sun damage also increases the rate of cataracts and causes premature wrinkling. Sun safety 101: Use sun protection at all times. Even on overcast days, up to 60 percent of the sun's rays penetrate cloud cover, and 85 percent of the sun's rays reflect off of sand, water and concrete. Minimize sun exposure from 10 a.m. to 4 p.m., the sun's strongest hours.