Hi,
Eyesight can change with age regardless of the fact that you study a lot. At the age of 21-22 it becomes stable. It might change after that but it does not change that much.
I wouldn´t worry about it much.
I have myopia. I started wearing glasses at age 12 with about -0.25 as I grew it became worse until I reached -4.25. I´ve had the same glasses for about 10 years now and it has only changed a bit (maybe -0.25 in these 10 years).
When you study, just look at distant objects once in a while for a minute or so and then continue. Also you can take short breaks to go walk a little bit, that helps.
Best
Arturo