Sour Nightmares

The opposite of "Sweet Dreams". Telling someone to have Sour Nightmares means you don't want them to sleep well or have good dreams. Instead, you want their sleep to be bad, unsettling, or even terrifying.

Guy 1: "I'm going to bed."
Guy 2: "Sour Nightmares."
Guy 1: "Don't you mean Sweet Dreams?"
Guy 2: "No."