>This "keep sleeping or wake up" should be entirely decided in the ISR.

It cannot be done like that. The ISR has to set a flag, and then return from 
interrupt, allowing the main program to run around a loop looking at the 
flag, to see if anything needs to be done. If nothing is required, then the 
main program puts the CPU to sleep. The ISR should never be putting the CPU 
to sleep. 

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