Our new dog has started barking at night. Any advice?

We got a second dog, Lilly, a month ago. She is a border collie mix. She has been great overall, but the last 4 nights she has started barking loudly in the early hours of the morning, in 15 minute stretches. As my wife puts it “I love her, but sometimes I want to punch her in the face”
(this is 3am grumpy talk, we aren’t violent)

Any theories about why she’s started to do this? Any recommendations about how to get her to stop?

Additional details:
We got Lilly from the Humane Society. They estimated that she’s 2 years old. They don’t have any information about her previous home(s).

Our first dog, Bella, is also a border collie mix.

Both dogs sleep inside in the kitchen at night.

Our neighborhood is very quiet. I don’t think she is barking at sounds she hears. Bella hasn’t been reacting to anything.