When your eyes are dry, they become irritated and inflamed. It’s that inflammation that causes the redness.
Computer Screens - Dry eyes tends to be more common in adults over 50, but with all the technology now days such as iPads, laptops, smart phones it's more common now to see younger people being affected who spend a lot of time staring at their screens. We don't blink much when we are looking at the various screens so our eyes are much more likely to get irritated.
A good rule of thumb to prevent this is to follow the 20-20-20 rule where every 20 minutes, take a 20-second break and focus your eyes on something at least 20 feet away.
Allergies - Allergens like pollen and grass can cause swelling and inflammation in your eyes, which can lead to redness.
Since allergies also tend to make your eyes itchy, most people tend to rub their eyes which causes even more inflammation and redness.
Certain Medications - Certain medications such as antihistamines sleeping pills, anti-anxiety meds, and even pain relievers like ibuprofen can potentially dry out your eyes and causes redness. The reason some medications can make your eyes dry and red is because they reduce blood flow to tissue in and around your eye.
Not Enough Sleep - Whether you stayed up late watching Game of Thrones or spent half the night tossing and turning, your eyes will probably look a little bloodshot in the morning. Your eyes need to be closed for an extended period of time for your tear layers to regenerate and replenish otherwise it leads to a loss of tears which leads to redness.
Drinking Alcohol - It’s not just the fact that you didn’t get home until daybreak but also because alcohol causes your blood vessels to relax which allows more blood to flow through the vessels in your eyes, so they look redder.
Smoking - Cigarette and cigar smoke causes the blood vessels in your eyes to constrict, which dries out the surface of your eyes. When your body senses the dryness and lack of blood flow, it tries to compensate by dilating your blood vessels and that will cause more redness.
In the long-term, the blood vessel constricting-effects of smoking (or exposure to secondhand smoke) can raise your risk for macular degeneration and cataracts—which can cause vision loss and even blindness.
Pink Eye - Pink eye, or conjunctivitis, is a viral or bacterial infection that can strike in one or both eyes. It’s also highly contagious and easily spreads from one eye to the other by just rubbing your eyes. If you think you have pink eye it's a good ideas to go see your eye doctor, who can examine your eyes to determine whether your pink eye is viral or bacterial and write you prescription.
Swimming - The salt in ocean water is drying, which leads to redness. Or if you swam in a pool it contains caustic chemicals, like chlorine, that actually kill off some of the good bacteria in your eyes—which can lead to irritation and redness.
You Popped a Blood Vessel - A popped blood vessel can happen if you were straining such as coughing intensely, if you slept in a weird position that put a lot of pressure on your eye or experienced some kind of trauma—like getting punched or being in a car accident. When a blood vessel bursts, blood gets trapped under the surface of your eye which can cause a bright red splotch, or make the entire white of your eye turn blood red. A popped blood vessel looks bad but they aren't actually serious and you will recover in about a weeks time.
Visine - It doesn't always get the red out. Weirdly, eye whitening drops can actually make your redness problem worse and you should generally AVOID these drops altogether. Products like Visine usually use ocular decongestants that shrink the blood vessels on the surface of your eye, making your eyes appear temporarily less red. The problem is, using these drops on a regular basis typically causes more irritation than they prevent and when you stop taking them, they cause a rebound inflammation, making your eyes appear even more red and agitated than before.
Over the Counter Eye Drops
Most eye drops contain preservatives that inhibit bacterial growth and keep them safer. But some people are sensitive or even allergic to these preservatives.
So keep in mind that these eye drops containing preservatives actually can cause irritation, redness and more dryness, rather than easing these symptoms.
I know when you go to Walmart there's a million choices of over the counter eye drops however, the best thing to do is have your Optometrist recommend any eye drops that you might need for your eyes.
Many Optometrists suggest using non-preserved (preservative-free) eye drops to avoid this problem. Most non-preserved eye drops come in unit dose tubes — meaning that, once they're opened, they should be used up or thrown away after one day. This is because preservative-free eye drops can grow bacteria, if they are kept too long and exposed to the elements.
If you are self-medicating red eyes it may only prolong the correct treatment or diagnosis of a potentially serious underlying condition and could cause issues to your sight down the road.