People assume space means something is wrong, so asking for it turns into building a case. You explain your mood, you apologize, you promise it is not about them. Sometimes you just need a few hours where nobody wants anything from you, and that is not a symptom of a problem. It is an ordinary amount of upkeep.

Say it simply next time: I need a quiet night, I will text you tomorrow. No diagnosis, no reassurance tour. Most people take that perfectly well, and the ones who demand a reason are usually asking for something else entirely.