aged Schnauzer has started barking in night - why?

by Tam
(San Antonio, TX, USA)

I have a 13.5 year old minature schnauzer named Max. About 2 years ago after picking him up from the vet following teeth cleaning, I noticed seizure activity. He has been on phenabarbitol daily and appears to be doing well in that regard.
For the past few months, Max has been barking in the middle of the night. No matter how many times we let him outside before bed to potty, his internal alarm arouses daily anywhere between 1 & 3am with a round of barking.
We look around and never see anything suspicious indicating the reason. Just wanted to know if this is normal for an aging miniature schnauzer? Any ideas on why he does this?
Is he becoming senile? Max is a loving old man who enjoys being with people.