Ah...mystery solved.

The imp has been getting up very, very early, lately.  As in, not long after six in the morning. 

I woke up from a bad dream, this morning, and just went ahead and got up.  The mp was already up, and looking very sleepy.  I asked him why he was up: did he just wake up, or did something wake him up?  He said something woke him, and it was outside.

His bedroom is on the northeast corner of the house.  We have a neighbor with teenage daughters--loud, screechy ones--on the north side. 

Those girls have been waking the imp, and the imp has been waking the pixie for company. 

I think I may need to talk to the neighbors.  And I need to see if I can find what the kids did with the white-noise machine, so I can try setting that up for the poor little guy.