Rhoda S. Narins, a clinical professor of dermatology at New York University Medical Center and the president of the American Society for Dermatologic Surgery, explains.

As we age, fat comes out of the space enclosed by the eye socket, called the orbit, and forms a puffy area under the eye. This fatty tissue can fill with water. Thus, it becomes more noticeable when water is retained, which can occur for a variety of reasons including eating too much salt, lying flat in bed, not getting enough sleep, allergies, and monthly hormonal changes. Dark circles under the eye occur for several reasons: The skin there is much thinner than skin elsewhere on the body and becomes looser under the eyes as we age. This very thin skin also sits on top of underlying purple muscle and blood vessels and thus appears darker. In addition, some people have hereditary pigmentation in this area.

Treating hollow space under the eye is straightforward and can be done by injecting a filler such as Restylane. After this procedure, the so-called tear trough is instantly softened and any pigmentation noticeably lighter as a result of putting something between the skin and the underlying tissue. Blepharoplasty (surgery) can be done to fix the fat pad under the eye or a carbon dioxide (CO2) laser can be used to resurface the skin, which tightens and thickens it as well as lightens the pigmentation.

Simple measures to reduce puffiness and darkness of under-eye circles include avoiding salt, getting enough sleep, treating allergies, sleeping on two pillows, raising the head of the bed and using cool compresses on the eyes. For hereditary pigmentation, CO2 laser resurfacing and bleaching creams are sometimes helpful.