Of course, after dark, they're going to be sleeping since they can't see those tempting feathers. My bet is that they're nibbling away at each other's butt feathers as it begins to be light enough for them to see, but not light enough to hop down from the perches.
When I first began my flock, I started out with two adult three-year old hens and soon after, got three six-week old chicks. Not long after, I noticed one of the adults had a perfectly shiny, bald butt, just as you describe your chickens. It was perfectly by chance that I was able to observe one of the chicks situating herself to nibble the feathers from the butt of this hen. It's been so many years that I forget all the details, but I did notice that it was a mistake to situate the perches as I had them
I got busy and figured out a better placement for the perches, and that solved the problem. It was a matter of removing the temptation and easy access to butts. During the day, there was no way those older hens' butts could be vulnerable to naughty chicks.
Meanwhile, you may be able to discourage further feather plucking by painting the pink exposed flesh with Blu-kote. For some reason, the sight of bare flesh stimulates the urge to pick. And Blu-kote is nasty tasting, too, but will help prevent infection if there are any skin lesions.