#include <avr/sleep.h>
boolean ledOn;
int ledPin = 13;
int intPin = 3;

int start = 0;

void setup() {
  ledOn = false;
  pinMode(intPin, INPUT_PULLUP);
  pinMode(ledPin, OUTPUT);
  //Serial.begin(9600);
  attachInterrupt(digitalPinToInterrupt(intPin), wakeUpNow, LOW);
  start = millis();
  digitalWrite(ledPin, HIGH);
  sleepNow();
}
void wakeUpNow()
{
  //Serial.println("wake up");
  digitalWrite(ledPin, HIGH);
  start = millis();
}
void sleepNow()
{
  //Serial.println("sleep now");
  digitalWrite(ledPin, LOW);
  set_sleep_mode(SLEEP_MODE_PWR_DOWN);

  sleep_enable();
  attachInterrupt(digitalPinToInterrupt(intPin), wakeUpNow, LOW);
  sleep_mode();

  sleep_disable();
  detachInterrupt(0);
}

void loop() {
  int now = millis();
  if (5000 < now - start) {
    sleepNow();
  } else {
    digitalWrite(ledPin, HIGH);
  }
}