So, if you add a static object (like a terrain piece or just a simple plane) into the game scene, it won't allow for objects resting on it to go to sleep. This can be seen in the example file by the cube landing on the added plane and not going to sleep. If you add a plane or engage the layer below the active one and play the engine, the cube will land on the 'native', non-added plane and go to sleep very quickly.
As a side-note, the added static object doesn't appear the same as a 'native' version of the same object in the physics visualization. The added static object's collision mesh has a red outline and white faces (like a dynamic object, apparently), as opposed to the normal red-outlined, green-faced static object appearance.
What this boils down to is that added objects don't allow others to go to sleep on them. It would appear that added objects themselves can go to sleep just fine, though.