Of course you can! It would be better, even. It's actually a very open question "How are you?" - Personally, I really like it when someone doesn't reply with the usual "Good" or "Fine", but actually answering the question. I noticed in Japan they teach it very straightforward "How are you?" - "I am fine, how are you?" But I would definitely encourage to reply to the question with truth. ^^ Note; most people will stay say "good" or "fine". Usually people ask "How are you?" out of politeness. Perhaps it's more friends/family that you could expect a more in depth answer from. This is all my personal opinion, other opinions may vary.

The standard replies to the question are Im fine , im ok , im good , im alright etc -- used when you reply, out of good manners., to the question which is made casually and also out of good manners , without expecting you to elaborate . But , if the person is close , and you wish to tell some truth, even if you answer in the negative ,it is totally ok . Then , maybe , you say : Im fine but i m feeling very tired .. Im fine except for the fact that im so tired because of work i have being doing .lately