I'm seeing places where, if it's attributed to Kafka, it's supplemented with a crack about "a chimney's sweep's Saturday night." To my mind, that doesn't make any sense, as I doubt Kafka gave a damn. A few searches inside complete works of Kafka books (keep in mind these are translations, so you need to look in a few of 'em) for the term chimney sweep
comes up dry.
Sounds too depressing to be Plato. Perhaps it's neither - there's tons of junk online attributed to this, that or the other author in order to try to give it some sort of perceived cachet.
I'd check a library if I were you.