Dogs can bark at sounds you can't hear.
Dogs bark if there are possible intruders nearby.
Dogs can bark due to lack of company.
Active dogs can bark if they don't get enough exercise.
Pain or discomfort can cause a dog to bark.
Isolation or lack of interaction can lead to barking.
A good routine can help reduce nighttime barking.
If barking continues, consult a vet or behaviourist.