How can people let their dog dark all night long?

Seriously, I had to talk to my neighbor about their dog barking several months ago. They were good and kept him inside at night for a few months but now he's back out at night barking from about 10PM to 3AM. Been going on for about 2 weeks now.

I'm going to speak with them again tomorrow. I'm worried I'll have to escalate this to a city. The complaint process says I need to get another neighbor involved. I'm beginning to document everything. There's one other neighbor I think might join me but failing that what's my next recourse? Do I have to sue?