They don't have to rebook you for free. Can you imagine if everyone was allowed to use some lame-@#$ excuse to get out of paying a change fee?

Alarm clock broken? Guess you should have bought a reliable one. It isn't the airlines problem if you fail to get to the airport in time, nor should it be.

Why not just do the honest thing, call the airline and tell them you want to fly out the next day? You may have to pay a change fee, but it is better than the airline cancelling your return leg for you not showing up.