The Scientist: Arthur Perry, M.D., YouBeauty Cosmetic Surgery Expert

The Answer: If you don’t want the girls to sag, then you’ll have to tuck them up before you tuck yourself in. It’s sad but true: Sleeping sans bra is the major cause of breast droop.We wear bras during the day to bolster our breasts against the pull of gravity, then we lie braless in bed for eight hours. Well, guess what, gravity works at night, too.The scientific name for droop is ptosis, a term that is often used to describe sagging eyelids. Ptosis of the ta-tas is a basic function of gravity, size and time. The downward force of gravity stretches the skin around the breast, and as we age, the skin’s natural collagen support system weakens, making it harder to bounce back.

Size matters, too. The larger your breasts are—whether due to genes, implants or pregnancy—the more they’ll droop. A and B cups needn’t worry too much, while those with Cs and up should make a habit of sleeping with support.

Rapid volume growth, from weight gain, pregnancy or implants increases sag potential. The dermis layer of your skin stresses and tears to compensate for the quick change—we know this as stretch marks. That stretched-out skin is more likely to droop.The best thing you can do is keep the girls in check whenever possible. There’s no evidence that sleeping in a bra has any negative consequences beyond a pinch or a poke here and there, so just make sure to find something comfy like a sports bra or a cami with a built-in.And don’t fret that pushing them together at night will give you cleavage wrinkles. That’s all about aging and sun damage, so you can put that myth to bed.