Best Way to Save Power

In the Nightingale example I understand most of it, but can anyone explain the following.

cbi(ADCSRA,ADEN); // switch Analog to Digitalconverter OFF
sbi(ADCSRA,ADEN); // switch Analog to Digitalconverter ON

In the void system_sleep() section , What does this do and why is it needed?

Also, I am a bit confused by the watchdog code.
Where do I set the time I want the watchdog to wait before waking up the Arduino?

void setup_watchdog(int ii) {

  byte bb;
  int ww;
  if (ii > 9 ) ii=9;
  bb=ii & 7;
  if (ii > 7) bb|= (1<<5);
  bb|= (1<<WDCE);
  ww=bb;
  Serial.println(ww);


  MCUSR &= ~(1<<WDRF);
  // start timed sequence
  WDTCSR |= (1<<WDCE) | (1<<WDE);
  // set new watchdog timeout value
  WDTCSR = bb;
  WDTCSR |= _BV(WDIE);


}