As I mention on my page about interrupts you can use pin-change interrupts on the Atmega328P to wake from the deepest sleep mode (SLEEP_MODE_PWR_DOWN).
const byte LEDLOOP = 8;
const byte LEDWAKE = 9;
// handle pin change interrupt for A0 to A5 here
// toggle LED
digitalWrite (LEDWAKE, !digitalRead (LEDWAKE));
} // end of PCINT1_vect
void setup ()
pinMode (LEDWAKE, OUTPUT);
pinMode (LEDLOOP, OUTPUT);
digitalWrite (A0, HIGH); // enable pull-up
// pin change interrupt
PCMSK1 |= bit (PCINT8); // want pin A0
PCIFR |= bit (PCIF1); // clear any outstanding interrupts
PCICR |= bit (PCIE1); // enable pin change interrupts for A0 to A5
} // end of setup
void loop ()
// flash to indicate we got out of sleep
digitalWrite (LEDLOOP, HIGH);
digitalWrite (LEDLOOP, LOW);
} // end of loop
This has the advantage of consuming very little power while asleep, and also waking on any pin (of an Atmega328P).
Running on 5V at 25°C you should only consume around 150 nA of current while asleep (on a "bare bones" board without USB chip, voltage regulator or power LEDs).
Other sleep modes may use more power but wake more quickly.