Wiktionary
vb. 1 (context idiomatic colloquial English) To go to bed; to go to sleep (especially at, or around midnight). 2 (context idiomatic colloquial English) Used to indicate a curfew, or the time by which one must depart.
Usage examples of "turn into a pumpkin".
All I know is that I'm going to study like a maniac this weekend, finish my midterms next week, go hear Travis sing at The Beanery on Wednesday, have a fabulous time with him, and run to the bus station by midnight to meet you guys and go back to Jacksonville for Thanksgiving-before I turn into a pumpkin.
What made you realize you might not turn into a pumpkin if you had sex more than once with somebody you actually gave a damn about?
If you didn't return by midnight, your pussy would turn into a pumpkin.
Later, Dana said reluctantly, 'I'm afraid it's time for Cinderella to turn into a pumpkin, darling.