Why does my dog keep being sick at night?

My first recommendation would be to seek the advice of a vet to rule out a medical cause. Acid reflux can cause nausea and vomiting on an empty stomach, and these symptoms are particularly apparent overnight since that's generally the longest period of time our dogs go without food. Another possible cause would be anxiety. After medical causes have been ruled out, try changing your bedtime routine to maximize your pup's comfort. Crate training or providing another safe, den-like space for your dog may help.

He your dog sick every night? When you say he is sick, what exactly do you mean, what happens and how much of it, what does it look like? How long has this been going on?