Ask your eye doctor to check for cataracts. Do not accept "we don't see anything"; chase them for an answer to your problem.

The "Posterior Subcapsular" type of cataract is apparently apt to form relatively quickly and be difficult to see; my wife went through 2 years of fruitless exams and glasses before figuring out the problem. Now she has her eyes back; thankfully.

Related: Most people over age 30 should see an ophthalmologist at least once every 10 years. An optometrist is generally fine for fitting glasses, but an ophthalmologist will provide a much more thorough eye exam (as well as updated RXes).

(Exception: If someone's vision is 20/20 uncorrected and they don't have any other eye concerns, this probably isn't needed… until they hit the all-but-inevitable presbyopia, likely around age 40-45.)

Not just cataracts. Have them check for macular degeneration, too, which if I understand correctly can also make your vision dark. (For that matter, have them check for glaucoma while you're in there.)

But I have a question. Is it just the monitor? Or do you notice the same thing with daylight? Room lights?

Mostly monitors. I do need glasses for day to day life, but I don’t feel like it’s dim out there or anything.

backlights do fade; perhaps look at a new monitor to compare?

See an eye doctor for one. Secondly you're dealing with perceptual issues and you should verify that your perception isn't faulty. See if you could find a light meter or use your phone in some way to gauge the brightness of your screen and of your environment. If you could look outside at a sunny day and you don't have to squint for a second because it actually hurts to see the brightness of the day then you might actually have a physiological thing going on.

How old is your monitor and laptop?

Older LCD screens primarily used CCFLs for backlights, and these were notorious for going dim after only a couple years.

All modern screens use LEDs for the backlight which are much more robust but still get dimmer over time. I know that my screens certainly have dimmed significantly since they were new, but not so bad as to be hard to read.

Environment plays a huge factor as well. All screens appear dimmer in the brightly-lit room as opposed to a dark or dimly-lit one.

IT happens with both my external monitor (4 years old) and MBP's internal display. I thought about keeping the room a tad darker than outside, but some people say it's not good in long term.

I get this. I often find myself thinking “why is this so dim?”. Try to turn up the brightness and it’s already at 100%.

Same thing is happening to me regularly.