Thank you @scalz! That did the trick. Finally I can debug the code with readable serial feedback. Ok @AWI so the example code is probably in need for some adjustments. Its kinda hard for a beginner like myself to learn the basics if also the example code suffers from questionable functionality. Or perhaps the functionality is ok but the code by itself is not clear enough to educate the masses. I'm still having trouble understanding whats really happening when using the sleep() call. In my newest revision I'm not passing any interrupt conditions other than a time variable but still its obvious that the sleep is interrupted by the interrupt defined in the setup section. And as such, perhaps also some timer functionality is running while in a MySensors sleep. Perhaps the API description is in need for some additional clarification. Note: I've also deleted the attach and detach calls from the loop() section as this is probably handled by the sleep function (or other hidden code).