MacBook Pro in safe mode when plugged in

My 2 month old MacBook Pro 13" (running 10.8.5) has a strange and annoying habit: When the power adapter is plugged in and I close the lid to let him fall sleep, after some hours (for instance: over night) it puts itself in safe mode, taking minutes to get alive again. Sleep mode without the power adapter attached works fine, even overnight it starts directly as expected.

Any thoughts?