Why does my dog bark at night?

Boredom. Dogs become bored easily when they have nothing to do, and will make their own fun. Barking at every sound they hear, joining in with the neighbor dogs in a group barking session, or just barking to let out energy are all reasons behind night barking. Loneliness.

Dogs bark at night because they hear something outside. Or in my case my lab barks when something is wrong with her such as she needs to potty. A dog I have boarded whined because she wanted water(we take it away at night at like 8 or 9 and wake up at 4).

Nighttime is when all the normal day to day noises stop. The outside ones, and the inside ones. Without this background noise a dog's sensitive hearing can pick up on noises it isn't normally used to hearing. So it barks at these unknown noises. Also it is better able to hear other dogs that are barking and respond to them as well.

They can hear more noises, it's harder for them to determine what is making the noises, and some guardian breeds are naturally more on edge at night. Some things that can help this are a white noise machine or playing classical music, and sticking to a routine at night, as well as making sure the dog gets enough exercise and stimulation during the day.

Your dog might be barking at night simply because he/she hears noises outside of the house (wind blowing, other animals, cars, etc). It is just their way of alerting you that something is disturbing them.