The tempation is still there ("hm... as long as I'm here, I should also write the mummble method, because I know I'll need that at some point..."),

I know this goes against standard XP guidance, but I find that writing that extra method for completeness sake is often in of itself a form of advanced test. A supposedly general solution that has in fact been tailored for a specific problem set/domain often passes all tests for that domain, but as soon as you push it out of its "comfort zone" you find it breaks. And this breakage sometimes could have happened inside the desired domain, but wasn't being tested for properly because the percieved scope was narrowed so tightly to "get the job done".

Leaving the tension unresolved overnight so that my mind keeps generating ideas and options helps provide instant focus, quickly overcome morning starting friction. And I'm not sleeping any worse than normal.

In some respect I consider you a lucky man. When I have unresolved tension left at the end of the session I cant get my mind off of the subject. I become nervous and irritable and poor company. I far prefer to end sessions with the completion of a module or its core functions or whatever. That way I _can_ fall asleep.