It sounds like a power issue. Technically, a computer can stay in sleep for forever, so long as it has the minimal amount of power that it requires to keep it's memory up. I can only reiterate what has already been said though, sleep is definitely not intended for that long.
What type of power source are you using? Surge strip, UPS, wall socket?
I keep my iMac, Mac mini (my home server), my linksys wrt610n router, and my Drobo all on a UPS. There are times when the voltage at my apartment fluctuates below 120v and the UPS is forced to kick in, although the power didn't technically "go out". You could have the same sort of problem.