Great question. Scientists just found out that tanning every other day makes your skin tan faster[ii]. The theory behind this shocking result is that your skin can only protect you in so many ways. They can use them to get a tan or to heal a sunburn. Yes, SPF could slow down the time it takes for you to tan, but it's much more likely to help you tan better and faster by letting your skin put all of its energy into melanogenesis, which is what gives your skin its color.