Well, depends on what you mean by awake. Do you mean the display is active (lit up)? If so, then that would be actually a low drain for that.
Now if you mean just that it is turned on, the screen is dark and you aren't actively using it, then it could be one of two things.
One, one or more of you apps is running still. Even if you are not actually using them, but they are open, then they are draining the battery. If you want to leave the phone in a state that this won't happen, then you need to actually close your apps. Not deactivate or anything, just don't leave them open in the background. You actually have to close the app, or it is still running.
Two, it is a bad battery. If it is this one, then you likely need to take it back...