Servo8Bit author here. I gotta admit, I didn't consider low power modes when I wrote this library. Once you attach the first servo ( by calling attach() ) the Servo8Bit library configures the timer (Timer1 by default) along with the compare match interrupt and then never touches it again. So when you detach all the servos, the timer is still left ticking and the compare match interrupt is still left active and continues to fire as usual, just with all servos detached, no pins are driven high or low.
What Nick said I think is good advice. I want to add to it and point out that after you wake up from sleep you need to turn timer1 back on by setting its prescaler back to the value it was before you disabled it and went to sleep. If you are using the default timer1, the code will look like this:
//create the variable, init it to zero.
uint8_t originalValue = 0;
//save the value of the Timer Counter Control Register 1
originalValue = TCCR1;
//turn off timer 1
TCCR1 = 0;
/* go to sleep */
/* Time passes, grass grows, tectonic plates shift, then suddenly... */
/* wake up */
//Restore the TCCR1 register to re-enable the timer.
TCCR1 = originalValue;
//reattach the servo that was previously detached
myServo.attach();