I'm not sure, but I think it isn't that important anyway..
An immovable object doesn't need as much processing as a moving object..
Only collisions with it are important, for it doesn't react to forces anyway..
A sleeping object still reacts to collisions and probably the "is sleeping" will be turned of immediately on collision..
So in a matter of speaking you could say an immovable object is always sleeping, but always awake is actually the same, for the calculations when awake or sleeping should be the same for an immovable object..