My six year old, very healthy indoor cat has, over the last week or two, increased his appetite significantly. Used to leave some food behind. Now gobbles it all up, and wants more. Shows no symptoms of associated ailments or other irregularities. He's perfectly normal otherwise. Is this a seasonal thing, whereby he senses that winter is coming, and wants to eat more? Not that he can feel it getting any colder, but maybe he is aware that the day length is getting shorter?