void setup()
{

    set_sleep_mode(SLEEP_MODE_PWR_DOWN);
    //do stuff so I can see that it just turned on

}

void loop()
{
    //do stuff so I can see that Im in loop

    //sleep and wakie stuff
    wdt_reset();
    WDTCR |= (1<<WDCE)|(1<<WDE);//enable change of prescalar
    WDTCR = (1<<WDIE)|(1<<WDE)|(1<<WDP3);//prescalar set to 4sec before overflow, and interrupt mode

    //all the sleep things:
    cli();
    sleep_enable();
    sei();
    sleep_cpu();
    sleep_disable();
    sei();

    WDTCR |= (1<<WDCE) | (1<<WDE);
    WDTCR = 0x00;
}