Not being able to sleep cause you're excited about something.

Post-game, eh? While there isn't one after the final boss is defeated, there's a lot of sidequests you might miss the first time through and there's also certain content you can only unlock after beating the game at least once. Abyss also has a lot of extra written materials about the characters that's not stated in the game and they're worth reading as they give insights into character actions and motivations.

The only other game I remembered being so excited to beat past midnight was Persona 3, appropriately so too since I finished the final boss before it was 1.00 am on the clock.