I work from home so I get to see my two cats all day. During the daytime, everything is fine. I feed them twice a day, strictly wet food. Both cats use the litter box throughout the day. However, over the past week or so, every time I wake up in the morning, I find that one of my two cats has puked somewhere. I have no idea which cat is doing it or why it's always during the night. Do I need to take my cats to the vet? Is this a normal / common phenomenon? What's the usual explanation?