Routine is the key.  If something is a habit, how tired you are or whether you are in the mood to do it doesn't enter into it.  You just do it.

As for how to get into the habit...start small.  Setting up a whole scene or animation can be overwhelming, especially if you're tired. So tell yourself you'll do just one small part.  Just create the character, say, or the pose, or the prop.  Or tell yourself you'll only work for 15 minutes, then you can quit. 

Chances are, you'll forget about the time and keep working.  Or if not, you can still accomplish a lot in 15 minutes a day.