Your tear film — the thin layer of moisture coating the front of your eye — is made up of three layers: an oil layer, a water layer, and a mucus layer. Each one has a job, and when any of them is disrupted, the surface of your eye dries out and gets irritated. That's dry eye. It's genuinely one of the most common eye conditions among people who spend eight-plus hours a day in front of a screen, and a lot of people write it off as "just tired eyes" and ignore it — which usually lets it get worse.
Two things compound each other in a typical Bangkok expat day. First, screens: when you focus on a screen, your natural blink rate drops from roughly 15 blinks a minute to somewhere around 5–7. Fewer blinks means the tear film isn't being refreshed and spread evenly across your eye as often, so it evaporates faster than it's replenished. Second, air conditioning: Bangkok's heat means AC runs hard and constantly — in condos, offices, malls, cars, co-working spaces — and that constant conditioning strips humidity out of the air. Low humidity accelerates tear evaporation directly, on top of the blink-rate problem. Sit under a vent that blows straight at your face for eight hours and you're compounding it further. Put screen-focused blinking and AC-dried air together for a full Bangkok workday, and it's easy to see why eyes that were fine back home start feeling gritty, red, and tired by early-to-mid afternoon.
A common mistake: reaching for "whitening" or redness-relief eye drops instead of proper lubricant. Many of these contain preservatives or vasoconstrictors that can make dry eye worse with frequent use, even though they make your eyes look less red in the short term. If your eyes are dry, what they usually need is a preservative-free artificial tear — not a redness reliever.