Does bright environment strain eyes more than dim?
I think the principle is "everything in moderation". Environment should be not very bright and not very dimmed. Also, the contrast should be moderate too, i.e. having bright screen in dark room is not good. Also, he type of work should not be very constant, i.e. you should sometimes change looking at the screen to looking in the window and similar.

Remember, that the light is required for stimulating the brain activity.