Cat urinating outside let box

I have a anxious cat. He has to be on Prozac to function like a normal cat and it has made a tremendous improvement. He does like to Lee outside of the litter box when I leave for more than a day. He only does it once I return though not while I'm gone. Lucky I only leave a couple times a year. But the last few weeks he has been wanting to urinate on my couch. But only between the hours of 4-6am. My daily routine has not changed at all so I can't figure out why he is doing this but only between those hours in the early morning. Any idea on why this is happening?

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