I've experience just this thing lately from a Mockingbird and all night long...same bird too. But it's starting to be very comforting to hear for me now because I know he is just doing what he does best.
Also I have grown very fond of this one bird for weeks now..always good to see him sitting on his favorite perch and he is doing ok.
We had an experience this summer with a Mockingbird singing all night.
We stopped in a town along the St Johns River to visit with a niece who lived there.
We parked our RV overnight (with permission) in a Wynn Dixie parking lot.
We were under a street lamp along the edge, next to a rather large Crepe Myrtle.
No sooner than we shut our eyes, this fellow started up with song.
He was quite remarkable. I had never heard so many riffs and variations from a single bird.
While feeling a bit irate with him keeping me awake, I was fascinated.
I remember from when I was a youngster in Florida, Mockingbirds could imitated the audible part of an Alligator's mating bellow.