#include <avr/sleep.h>

#ifdef __AVR_ATtiny13__

  #define SLEEP_FOREVER  128
  #define SLEEP_016MS    (period_t)0
  #define SLEEP_125MS    (1<<WDP1) | (1<<WDP0)
  #define SLEEP_250MS    (1<<WDP2)
  #define SLEEP_500MS    (1<<WDP2) | (1<<WDP0)
  #define SLEEP_1SEC     (1<<WDP2) | (1<<WDP1)
  #define SLEEP_2SEC     (1<<WDP2) | (1<<WDP1) | (1<<WDP0)
  #define SLEEP_4SEC     (1<<WDP3)
  #define SLEEP_8SEC     (1<<WDP3) | (1<<WDP0)

#endif /* ifdef __AVR_ATtiny13__ */


  // without that empty declaration, I got some problems with my other code. Maybe you don't need that.
  ISR(WDT_vect) {
  }


  void sleepNow(byte b) {
    {
      ACSR |= (1 << ACD); //Analog comparator off
      ACSR = ADMUX = ADCSRA = 0;
    }

    if (b != SLEEP_FOREVER) {
      WDTCR |= b;    //Watchdog
      // Enable watchdog timer interrupts
      WDTCR |= (1 << WDTIE);
    }
    set_sleep_mode(SLEEP_MODE_PWR_DOWN);

    cli(); // No interrupts; timed sequence
    sei(); // Enable global interrupts or we never wake
    sleep_mode();
    sleep_disable();
  }


  // calling in loop() for example: sleepNow(SLEEP_1SEC);