TIL about energy refresh mechanics
This is one for everyone like me who hyper-optimizes every minute of this game. This discovery comes in two parts:
If you over-exert yourself during a day, you will not regain all your energy when you sleep that night--only half. Your energy gain is also decreased if you stay up past midnight, getting worse the longer you stay up.
HOWEVER! If you leveled up in any skill during the day, then your energy will be fully refreshed that night anyway, no matter how late you stayed up and even if you were over-exerted. This is the part I didn't know, and I have been using it to my advantage since I learned it.