An interesting anecdote for hard contact lens wearers:

I was told years ago that my vision is too bad for soft lenses, and have been wearing hard contacts since I was in high school. Hard lenses are not available in extended-wear varieties (a concept I'm not comfortable with to begin with, even in soft lenses), so I never left my contact lenses in my eyes for extended periods of time on purpose, and only rarely wound up, by accident, falling asleep with them in (everybody does this from time to time). I would, however, wear them for 16+ hours every day because I can't see without them and I hate, hate, hate, wearing glasses.

Every optometrist warned me to take my lenses out every night. It's not good to leave contact lenses in your eyes for days at a time if they're not designed for it.

What nobody told me was that I should be getting them replaced every year.

When I was wearing glasses, I had gotten used to my vision getting worse on a yearly basis. I would need a new prescription every time I went to see the eye doctor. When I got my first pair of contact lenses, things seemed to level out and my vision stabilized. So I didn't go see the optometrist as often anymore, usually only if I lost or broke a lens and needed a new pair.

This means that for the last four years I had been wearing the same pair of hard contact lenses. Apparently, this was not good for my eyes.

According to my new optometrist, over long periods of time, the chemicals and other biological nastiness of your eyeball will warp and deform hard contact lenses. Continuing to place them in your eyes will then warp and deform your corneas. The process was so slow that I didn't even notice until it was well under way, when I started feeling like something was in my eye from time to time but couldn't find it or couldn't wash it out. Fortunately for me, it turns out this is only temporary, and although it can take up to a year for the eye to pop back into shape, it will eventually do so.

As a side note, a friend of mine got some new contact lenses recently that were supposed to do this on purpose. The theory is that, for a person with a weak prescription, sleeping with hard contact lenses specially shaped to deform your eye in the right way, carefully measured and monitored by the optometrist, will improve your vision. It takes several days for the reshaping to take place, and if you stop, over the course of a couple of weeks your eye will return to its normal shape. The process was too uncomfortable for him, though (not everyone is good with hard contact lenses, and I would imagine even fewer would be good with intentionally malformed ones), and eventually decided to just wear his glasses.

The good news was that contact lens technology had advanced since I was in high school and they now make soft lenses in my prescription. The bad news was that over the course of last year, I had to get several different pairs of contact lenses (thank goodness for my vision plan through work) because my eyes were reshaping themselves and a pair would only fit correctly for a while. The first pair lasted a couple of weeks, the second a month, the third a couple of months, and the fourth about six months. This brings me to January, 2008, and I have a doctor's appointment tomorrow to get what I believe will be my final pair of contact lenses.

Wait a minute, your final pair? Haven't you learned anything from all this? You need to go back next year and get a fresh, new pair.

Not necessarily, because I'm sick and tired of all of this already. The doctor said I'll need to wait one year after my eyes have reshaped, and once we're certain everything is back to their normal conditions, I'm going to get laser eye surgery and correct my vision once and for all.