But surprise surprise: Researcher found that neither diet nor exercise, but only poor sleep, was associated with "transepidermal water loss"—a decrease in skin hydration. Yes, turns out "beauty sleep" is legit.

As dermatologist Marina Peredo, M.D., explains, "dry skin can appear more aged." Who wants that? Here's what she suggests before bed to keep skin plump even when you can't catch a full night's rest.

They're never great for your skin—heat dries it out—but they are particularly important to avoid pre-sleep because your body temp rises overnight and the double whammy of heat can parch skin. Need a rinse? Jump in at least an hour before you snooze.

Transepidermal water loss slows in humid environments. So if it's a dry time of year (like winter) or you simply live in a dry climate, flip on your humidifier at night to add moisture to the air and, ultimately, back to your skin as well.

With a product that sloughs and hydrates, the former makes the latter more effective. WH editors love Neutrogena Rapid Wrinkle Repair Night ($17, amazon.com). It combines exfoliating retinol with hyaluronic acid, which draws water into skin.