지치다 is being worn down and out of energy — drained, not sleepy. -(으)면 gives what does it: 2시간씩 운전하면 지쳐요 'driving two hours at a stretch wears me out', 매일 반복이 되면 지칠 수 있잖아요 'repeating it every day can wear you down'. It often follows -다 보면 'if you keep doing it', placing the exhaustion at the end of a long stretch rather than in one blow.