They are both nouns, but the gerund, “sleeping” refers to the action of the verb, and implies that you are the one who sleeps (in the afternoon). The noun, “sleep”, is more general, so it doesn’t really make sense in this sentence. The infinitive, “to sleep”, could be used and would have practically the same meaning as the gerund. (The gerund sounds a bit more factual than the infinitive, which may sound just a bit theoretical in some contexts.)