A daytime nap feels like a small mercy. Twenty minutes on the sofa, a quick reset, and you wake sharper than before. Yet of all the things 23,000 middle-aged adults told researchers about how they slept, regular napping turned out to be the habit most stubbornly tied to a particular kind of wear and tear deep inside the brain. Not snoring. Not nodding off at your desk. Napping. The finding comes from a team at the University of Arizona, working with colleagues at the University of Southern...