Having one or both eyes become red is so common that basically everyone will experience it at one time or another. It may range in severity, from several enlarged blood vessels in the eye, or it may look like a pink or even bright red which entirely covers the sclera. Everyone has blood vessels in their eyes, and you can always see them if you take a close look at the eye. However, with the case of red eyes, the appearance is considerably more noticeable because these ordinary blood vessels become swollen.
Generally speaking red eyes are not a cause of concern, and are usually just from something completely benign and normal. However, if accompanied by other symptoms, red eyes may pose a slight problem and in some rare cases they could even be indicative of some emergency. It all depends on why the eyes are red and if there are other issues as well.
Dry eye syndrome is a somewhat common and fairly harmless cause for red eyes, but it should still be managed with treatment. When the tear glands in the eyes can't produce enough tears, or the proper quality of tears, the eyes are unable to stay lubricated. This leads to irritation, inflammation, and redness.
If you are prone to allergies then red eyes are a very common symptom. Histamine is released to fight off allergens like dust, pet dander, and pollen, and the histamine is what causes the blood vessels in the eyes to enlarge. Of course this is generally just an irritation and does not usually indicate anything serious, but there are medications you can take to help with your discomfort.
If the eye becomes irritated the blood vessels in the eyes will no doubt react. Contact lenses can irritate the eyes and often cause red eyes, especially if you don't wear them correctly or you wear them too often. They may also lead to an eye infection which could be the culprit of the red eye or eyes, and if so this could be somewhat serious. Contacts may also make dry eye syndrome worse. To avoid problems keep lenses clean and replace them when necessary.
If your eyes become fatigued or tired after staring at a single point for long periods of time they may burn and become red. This is common with computer vision syndrome because many people use computers in this day and age. When working or playing on a computer you automatically blink less often which creates dry eyes. Frequent breaks are the easiest solution to this problem.
Pink eye is a type of infection which causes one or both eyes to become red. It is most prevalent among children and usually not too serious. Injuries to the eye may also cause eye redness. Swimming, smoking, some drugs and alcohol, pregnancy, cold or flu, and even whitening eye drops may also cause red eyes. There are a few more serious problems which can cause red eyes. Corneal ulcers, ocular herpes, uveitis, and glaucoma, may also be a part of red eyes. If there is pain, sensitivity, or discharge, it is always a good idea to see the doctor.