In some household, it may seem that when the sun sets, a dog goes on automatic alert. Is there a noise that is alarming your Chihuahua? Dogs have incredible hearing and will bark at things that we will never pick up on.
When your Chihuahua wakes you up at night, listen carefully. Is there a constant but soft noise? Could there be squirrels running along the roof? Perhaps a neighbor's TV, maybe a car driving by?
Many times a car will alarm a Chihuahua; it may be a neighbor who comes home at just about the same time each night and waking up your dog...some noises can drive a dog crazy
The way to deal with this is to let your dog know that you hear the noise also. Say in a strong tone, "Yes, I hear the noise, yes it is a car." Your dog, of course, will not know what you are saying, but dogs understand a lot of what you say by the tone of your voice.
Once you let your Chihuahua know that you also sense the noise, show him that there is no reason to be concerned. Pat your dog and in a gentle and relaxed voice, say something to the effect of, "It's okay, there's no reason to bark, everything is alright"
You can also repeat this but lower your tone each time, until you are whispering calmly; thus showing your dog that if you think everything is okay than it truly is.