Sleep and Wake Arduino according to LDR interrupt
I want to replace the conditional check with interrupt.


Why? If it is awake it might just as well be testing the pin as doing anything else.

If you make it an interrupt, in the ISR interrupts will be off. Then if you go to sleep it will never wake.

So, your current design is the correct one, except:

attachInterrupt(0,wakeUpNow, RISING); function


What is function doing there?