I've been meaning to do a deep dive on sun exposure, specification with respect to base tans. As I understand it, conventional wisdom is that base tans don't provide enough value to offset the risks, and that you should just use sunscreen any time you're in the sun.

Generally the way this is explained is that a base tan provides ~4 SPF of protection, which is far less than the recommended 15-30 SPF of sunscreens. I believe I read once that 4 SPF is only good for maybe 15 minutes of protection.

What I haven't seen explained (and what I want to learn) is how the body's "sun timer" resets. If I'm in the sun for 15 minutes and then go in the shade, how long does it take before I can spend another 15 minutes in the sun? If I'm hiking in a forest, I wouldn't be surprised if 15 minutes of protection is enough to completely avoid sunburn, since you're constantly going back and forth between sun and shade.

Another thing I want to clarify is mutation risk vs sunburn. Does a base tan protect you from mutations at all? I'm not actually sure.