Decrypt hidden message and find solution

Following an especially angry argument, Mr. and Mrs. Smith went to bed not speaking to each other. Needing to arise early the following morning, Mr. Smith left a note on his wife's bedside table that said "Wake me at six."

An exasperated Mr. Smith awoke at ten the following morning and rolled stiffly out of bed to see a note on his bedside table: "It's six, you bum! Get out of bed!"