Nothing happens to an input pin when the chip goes into power down mode.
What are you using to wake up the chip every 8s, the watchdog or something else? Why not use a pin change interrupt to wake up the mcu on a low to high transition of the PIR output? Post your code.
I'm using the watchdog. Can't use any of the interrupts available cause I'm already using them for other events... Every single pin on the Atmega 328 is already used (some with more than one task). Gonna start fiddling with other models for the next project cause the 328 is limiting.
1. You can still enable the pin-change interrupt on the input pin you are already using.