The sun is good for you. Our bodies need the sun to make Vitamin D which is essential to our health.
However, it's really good for your paper clay if you need to get it to firm up quickly. At full sun (especially in the hot Southern California summers), we receive about 100 watts of solar energy per square foot per hour. (1)
All clays can use the sun's solar power to firm up, but with paper clay, you don't have to worry about the clay going past leather hard. In fact, even if the paper clay is way, way dry for conventional (traditional) joining, it is not an issue. Paper clay with bond with itself with the correct joining techniques. With a traditional clay, once you go past this "leather hard joining window," you run into risk of your joints coming apart or the joints not even taking hold.
There are so many times I've said to myself, "Thank goodness I'm using paper clay for this project."
Using the sun's benefit is part of "forced drying". Here's the link to an earlier post of mine about this subject.