I'm looking for a good lightweight program which forces me to have my eyes relaxed from time to time after working with computer.
It is preferred if it blacks out the screen for a while from time to time, e.g. every 30 mins for about 5 mins.
Taking care of the eyes while working computers is the concern of the question, not exercising on the desk at the office.
It is highly suggested if it provides some exercise for the eyes — and not for the whole body — in the breaks.
It should run on Windows. It should be lightweight so that it runs well on startup. It should be free of charge (it doesn't matter whether it is open source or not).