The skin looks a little different. Sleep feels less restorative. Energy that used to be reliable starts behaving unpredictably. These changes tend to get attributed to stress or aging. The explanation is usually more specific than that — and more treatable.

More people are diagnosed with skin cancer than all other cancers combined — and rates are rising, especially in women. Yet most of us are still guessing at SPF numbers and skipping reapplication. Here's what the research actually says about how sunscreen works, how much to use, and where high SPF can backfire.