The notes from Nick Gammon has this mention …
[http://www.gammon.com.au/forum/?id=11488](http://www.gammon.com.au/forum/?id=11488)15/Dec/2013 : We now believe that all external interrupts (not just LOW ones) will wake the processor from sleep. This seems more consistent with the fact that pin change interrupts can wake the processor.
I’m not positive that FALLING will resolve this, but I think it’s worth a shot. Meanwhile I’ll stare at your code a bit more to see if anything else jumps up.