As the Earth rotates, the Sun appears to rise over the horizon. As its rays become more and more directly overhead, there is less distance for them to travel through the atmosphere in order to reach us. (This longer travel is what creates wonderful sunrises and sunsets; light is refracted at a greater rate, allowing us to see a variety of frequencies and thus colors.) Keep this in mind if you like to tan. It’s best (i.e., healthiest) to catch your rays in the morning and evening since less ultraviolet light can reach you.
And, as always, don’t believe the quacks who say sunscreen is bad for you. If you’re tanning in the middle of the day, wear it. Cancer is bad, ya know?