As far as I know, leJOS 0.9.1 will come with a firmware, which puts the NXT into a deeper sleep state, if there's nothing to do. So if all Threads are sleeping, the NXT's CPU is sleeping as well. Also, it has been observed that putting the motor in float state consumes less energy than having the motors in break mode. And I believe the little AVR co-processor which assists the big ARM CPU cannot be suspended as well. Even though enhancements were made, the NXT will probably not last all night. As far as I'm aware, there is no low-power wakeup timer inside the NXT. Also, I wonder why you think that using an (efficient) PSU is worse than charging the batteries the next morning.