Eye bags are caused primarily by weakening of the tissue structures and muscles supporting the eyelids. This weakening causes the skin to sag and allows the fat usually around the eye to shift into the area below the eyes. Fluid can also accumulate below the eyes, leading to puffiness or swelling. Key factors that cause or worsen eye bags include aging, fluid retention (especially after waking or salty meals), lack of sleep, allergies, smoking, genetics, and certain medical conditions like dermatitis, renal disease, and thyroid eye disease.

Additional contributing factors can be thyroid problems, volume loss in facial fat, stress, and lifestyle choices such as excessive salt and alcohol intake. Environmental factors like sun damage, allergies, and exposure to pollutants also play a role. Stress affects hormones and skin structure, making eye bags more noticeable.

In summary, eye bags result from a combination of aging-related tissue changes, fluid retention, lifestyle habits, genetics, and underlying health issues.