cats

1. Is he peeing normally? If you suspect he's not peeing, or is having trouble peeing, take him to the vet immediately.
2. Is he drinking water normally? If he's not drinking water, take him to the vet immediately.
3. Is he eating normally?

If he's doing those three things, it's probably OK to observe him for a couple of days and see how it develops. If he keeps it up, I'd take him to the vet.

We once had a cat that inexplicably started yowling in the middle of the night. Did not appear to be in any distress, but would not stop. In the morning, we discovered that a water heater had burst and we had 6" of water in the basement - where her litter box was. We've had them begin yowling at night, because they'd "caught" their toy and were letting everyone know how great a hunter they were. We've certainly had them yell when they caught scent of another cat through the window. Once, we had once start every night, the same time... turns out we had racoons visiting the porch, and our cat did not like the looks of them at all, even through the glass sliders. But, once, we had one take up yowling and prowling at night, and that turned out to be lukemia.

The peeing thing is super important; beyond that, most things that have no obvious injury, you can observe for a couple of days.