Not lose sleep/lose no sleep over something Definition 1 of 2Pro Tip 1/3Emotional Detachment Shows you're unaffected emotionally by an event or situation. She didn't get the promotion, but she's not losing sleep over it. Pro Tip 2/3Informal Expression Used more in casual conversation than in formal writing. We lost the game, but we're not losing sleep over it. Pro Tip 3/3Positive Attitude Implies a positive or resilient attitude in the face of problems. The deal fell through, yet he's not losing sleep over it.