Yesterday’s morning expression on awakening (with a need to answer the call of nature) was not exactly a name, but, well, the NP the call of nature. That led to the product Serutan — that is a name — and, in another direction, to the PP against nature, which I’ll reserve for another day.
Basic dictionary work. From NOAD2:
call of nature used euphemistically to refer to a need to urinate or defecate.
A need to urinate or defecate. Often used with answer: He left the room to answer the call of nature.
Idiom dictions are roughly similar, and some offer nature’s call as an alternative.