Hiding a lot

Why do cats start hiding or sleeping in closets more? Should I be concerned. He doesn't usually do this but the past couple of days he does all the time, he still eats but acts normal although does seem sleepier than usual. It could be this heat wave we are coming off of here in California too.

I know when it gets hot in my house, my cats tend to sleep in places they normally wouldn't, like under my bed. If the rest of his behavior seems the same, then it is probably just the heat.

I'd keep an eye on him - especially his stool and if he is peeing as usual. He may just be warm but the hiding doesn't sound right. Usually if they are very hot they'll find a cool place like a bathtub, sink, ceramic floor, etc.