Signs and symptoms, which usually affect both eyes, may include:

• A stinging, burning or scratchy sensation in your eyes
• Stringy mucus in or around your eyes
• Sensitivity to light
• Eye redness
• A sensation of having something in your eyes
• Difficulty wearing contact lenses
• Difficulty with nighttime driving
• Watery eyes, which is the body's response to the irritation of dry eyes
• Blurred vision or eye fatigue

Tear glands and tear ducts

Dry eyes are caused by a lack of adequate tears. Your tears are a complex mixture of water, fatty oils and mucus. This mixture helps make the surface of your eyes smooth and clear, and it helps protect your eyes from infection.

For some people, the cause of dry eyes is decreased tear production. For others it's increased tear evaporation and an imbalance in the makeup of your tears.

Decreased tear production

Dry eyes can occur when you're unable to produce enough tears. The medical term for this condition is keratoconjunctivitis sicca (ker-uh-toe-kun-junk-tih-VY-tis SIK-uh). Common causes of decreased tear production include:

• Aging
• Certain medical conditions, including diabetes, rheumatoid arthritis, lupus, scleroderma, Sjogren's syndrome, thyroid disorders and vitamin A deficiency
• Certain medications, including antihistamines, decongestants, hormone replacement therapy, antidepressants, and drugs for high blood pressure, acne, birth control and Parkinson's disease
• Laser eye surgery, though symptoms of dry eyes related to this procedure are usually temporary
• Tear gland damage from inflammation or radiation

Increased tear evaporation

Common causes of increased tear evaporation include:

• Wind, smoke or dry air
• Blinking less often, which tends to occur when you're concentrating, for example, while reading, driving or working at a computer
• Eyelid problems, such as out-turning of the lids (ectropion) and in-turning of the lids (entropion)

Imbalance in tear composition

The tear film has three basic layers: oil, water and mucus. Problems with any of these layers can cause dry eyes. For example, the oil film produced by small glands on the edge of your eyelids (meibomian glands) might become clogged. Blocked meibomian glands are more common in people with inflammation along the edge of their eyelids (blepharitis), rosacea or other skin disorders.

Factors that make it more likely that you'll experience dry eyes include:

• Being older than 50. Tear production tends to diminish as you get older. Dry eyes are more common in people over 50.
• Being a woman. A lack of tears is more common in women, especially if they experience hormonal changes due to pregnancy, using birth control pills or menopause.
• Eating a diet that is low in vitamin A, which is found in liver, carrots and broccoli, or low in omega-3 fatty acids, which are found in fish, walnuts and vegetable oils
• Wearing contact lenses

People who have dry eyes may experience these complications:

• Eye infections. Your tears protect the surface of your eyes from infection. Without adequate tears, you may have an increased risk of eye infection.
• Damage to the surface of your eyes. If left untreated, severe dry eyes may lead to eye inflammation, abrasion of the corneal surface, corneal ulcer and vision problems.
• Decreased quality of life. Dry eyes can make it difficult to perform everyday activities, such as reading.