BACK when Lisa Barr wore a size 34B bra, she didn’t know from cleavage wrinkles. But soon after she got breast implants in 1999, augmenting her measurements to a 36C, she started waking up with thick lines on her chest where one breast had fallen against the other as she slept on her side.

“The wrinkles would stay even when I was getting dressed,” said Ms. Barr, 40, a litigation paralegal in Rochester. Nor did they diminish as the day progressed. She could not help wondering why she had gotten the implants if she was going to have to cover the areas around and above them.

The skin just below the neck can reveal a woman’s age and skin-care history just as easily as her hands can. Cleavage wrinkles are deep, vertical creases caused by hours spent sleeping on one’s side, where gravity forces the top breast to bend farther past the body’s midline than it should. The lines can also be caused by sports and push-up bras, which smush the breasts together and are often worn for hours.