Bags underneath the eyes are commonly associated with fatigue. But research has shown that allergies and clogged sinuses may play a role in the dark circles you get under your eyes.

The Mayo Clinic reported that allergies can create dark circles under the eyes, and a recent video on the popular YouTube channel SciShow explained why.

For starters, the skin underneath your eyes is very thin, so there isn't much space or fat to hide your blood vessels, which can dilate when you have allergies.

"Allergic reactions can trigger your immune system to release these compounds called histamines, which cause blood vessels to dilate," the video said. "Plus, itching and rubbing your eyes can make the puffiness worse. So besides the dark circles from built-up blood and inflammation, puffy eye bags can also mean dark shadows."

Nasal congestion can also cause bags under your eyes.

That's the conclusion of Kris G. McGrath, a professor of allergy and immunology at Northwestern University:

"Puffy eyes and dark circles around the eyes are caused from nasal and sinus congestion interfering with blood flow through the veins around the eyes," McGrath told Everyday Health. He added: "Treatment involves avoidance and medications to reduce the inflammation in the nose, sinuses, and eyes. Typically topical intra-nasal steroid or antihistamine sprays are used often with an antihistamine and mast cell stabilizer eyedrops; they are most effective beginning two weeks before the pollen arrives."

The Mayo Clinic recommended rinsing out your sinuses.

"Extra zzzs may not cure those dark circles under your eyes. One common cause is nasal congestion. When your nose gets congested, veins that usually drain from your eyes into your nose become widened and darker. Rinsing your sinuses with a saltwater solution (mix 1/4 teaspoon sea salt with 2 cups warm water) or over-the-counter saline spray can help relieve nasal congestion and your dark circles."

Allergy relief medicine can also help decrease the puffiness under your eyes.

"If you have allergies, which play a role in puffiness since they contribute to the fluid build-up under the eye, be sure to take allergy relief medicine to minimize the amount of puffiness you might be experiencing," dermatologist Doris Day told Cosmopolitan.com.

Watch the full SciShow video below:

[H/T Lifehacker]