Can't wake up from sleep mode on arduino.

Hello,

I'm trying to minimize the power consumption on my attiny85 as it will be connected to a battery and I found that if you shutdown the ADC and put it to sleep mode it only consumes about 1uA wich is good so I added the following elements to my code:

#include <avr/sleep.h>

#define adc_disable() (ADCSRA &= ~(1<<ADEN)) // disable ADC (before power-off)
#define adc_enable() (ADCSRA |= (1<<ADEN)) // re-enable ADC

void enterSleep(void) // Function that puts the microcontroller to sleep
{
sleep_enable();
sleep_cpu();
}

void setup() {

adc_disable();
set_sleep_mode(SLEEP_MODE_PWR_DOWN);

}

void loop(){
if(mySerial.read() == 'g'){

for(int i = 0;i <= 4;i++){
Do something
}
enterSleep();

}

}

when I send the first 'g' it doe's ok but when I send another 'g' it doesn't respond assuming it's sleeping.

I tried that code on my arduino uno and I dont know what is the problem.

Do I have to wake it up or there is another method to use it?

Thanks for your time.