'Sad boy hours' is a self-aware phrase for late-night melancholy — listening to emo or heartbreak music alone, scrolling, ruminating on regrets. The phrase emerged from American rappers like Drake and the 'sad rap' subgenre of the 2010s. If your teen jokes about being in 'sad boy hours', they are being self-aware about their mood. Worth gently checking in if it sounds more serious than playful, since the framing can sometimes mask real distress.