For the face, dermatologists stress the importance of thorough cleansing and makeup removal. "Cleanse for one to two minutes using fingertips or a very soft washcloth," says Los Angeles dermatologist Evan Libow. "No squeezing, rubbing, or using abrasives is necessary." It's also essential to apply a moisturizer, perhaps a heavier one than is used for daytime. But "it doesn't need a sunblock," says New York dermatologist Diana Bihova. "Obviously, an SPF is of no benefit at night. It's like sleeping in your shoes."

Both dermatologists agree that women who are battling dry or aging skin should use a moisturizer with alpha hydroxy or glycolic acid. "They correct dry skin by normalizing the way epidermal cells shed," says Bihova. But Deborah De Piano, who calls herself a paramedical aesthetician (whatever that means) with the Camden Surgery Group in Beverly Hills, warns that many people are not using the acids properly—particularly when they get them in prescription strength from their doctors. "The people selling this...