Same happens occasionally on my mac. And even if it doesn't stay on, it wakes up often enough to drain >50% overnight. My only machine to reliably sleep, wake up and power manage is Linux.
Did you turn off "powernap" in settings? By default, a mac would wake up to download mail, run backups, and do other things in the middle of the night.
I've experienced this with Mac laptops that I've got loads of stuff running on, but not on stock configurations, so I suspect it's some piece of software rudely waking it up every so often for its own inscrutable reasons.
