1) If you are only reading 1 channel, you don't need any acquisition time.

2) When you put a PIC to SLEEP, all program execution STOPS.
Some peripherals may continue to operate, but the program will not run again until it wakes up.

3) When you put a PIC to SLEEP, the main oscillator is shut-down.
If you are running TIMER0 from the system clock, it too stops during sleep, and your 8mS periods will be extended.