It means to have no time to rest because you're awake (from hard work or whatever) during the day and night. It's figurative in that once half the candle is burnt out from things done (like working) in the night, it's turned over and the other half burns for things done during the day. "Julie is under a lot of stress from school at night and work during the day. She's been burning the candle at both ends and it's not good for her health."

It means working (or partying) very hard or for long hours, and not getting enough rest or sleep. This usually happens during final exam week. (And the partying after final exam week :D)

@Elina : Thanks so much for posting Craig's answer too... I like your attitude :-)

Craig's Answer was: "Burning the Candle at Both Ends! It means working too hard, doing too much. And it suggests that your health may suffer because of it." Thank you Craig for your help,buddy!