I get plenty of sleep, so what's with these dark undereye circles?

A lack of zzz's isn't the only potential cause of the shadows under your eyes. One common culprit: seasonal allergies. Constant congestion can affect blood flow to your sinuses, making the vessels under your eyes swell. This, in turn, brings on that purplish hue and what some experts call allergic shiners—enlarged veins showing through the delicate skin under your eyes. If you've also got itchy, watery eyes on account of allergies, rubbing them can make your shiners appear worse because it adds to the irritation in the area. An allergic reaction to any foundation, eye cream or other skin product can have a similar effect.

RELATED: 11 Weird Things That Make Seasonal Allergies Worse

To solve the problem, focus on nixing your allergy symptoms. You can soothe itchy, irritated eyes and congestion with antihistamines and nasal sprays. Be sure to take your allergy medication regularly to stay ahead of reactions. Your doctor can help you do a skin patch test to make sure that the issue isn't your beauty routine.

Dark undereye circles are also a common symptom of eczema, a chronic skin condition that usually starts in childhood and causes red, itchy rashes. These flare-ups are brought on by specific triggers, such as irritants in certain soaps, fabrics and even foods. The repeated inflammation can lead to darkening of delicate eyelid skin due to rubbing and lack of moisture. (People with eczema often struggle with seriously dry skin.) So it's very important that you moisturize daily with a gentle eye cream that doesn't contain any of your triggers. If your eczema is flaring up more than usual, ask your dermatologist about prescription creams that can help calm your skin and possibly brighten your eyes.

RELATED: What's That Rash?

Finally, as you age, the skin under your eyes becomes thinner and loses collagen, making it more translucent and allowing veins to show up more. (Sadly, some people, especially those of Mediterranean or Middle Eastern descent, are more prone to undereye circles, as they're an inherited trait.) An eye cream that contains retinol can help boost collagen production and reduce dark circles. And always wear sunglasses outdoors and slather on your SPF 30 every day, as UV radiation can further thin your skin.

RELATED: 14 Lifestyle Changes That Make You Look Younger

Health's medical editor, Roshini Rajapaksa, MD, is assistant professor of medicine at the NYU School of Medicine and co-founder of Tula Skincare.

Meet Dr. Raj at the Health Total Wellness Weekend at Canyon Ranch in May 2015. For details, go to Health.com/TotalWellness.