The sketch implements sleep enabled NRF24 based sensor node interfacing with Attiny85 in 5 pins configuration. It can only be woken up by RESET.

Attiny85_NRF24_Tx_5Pin_Sleep.ino

/*
 * @File: Attiny85_NRF24_Tx_5Pin_Sleep
 * @author: Tamojit Saha <https://tamojitsaha.info>
 * @Description: The sketch implements sleep enabled NRF24 based sensor node 
 *               interfacing with Attiny85 in 5 pins configuration. 
 *               It can only be woken up by RESET.
 *               
 * @license: Creative Commons License (CC-BY-NC-SA 4.0) 
    This work is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.
    To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-sa/4.0/
    or send a letter to Creative Commons, PO Box 1866, Mountain View, CA 94042, USA.
*/


#include <RF24.h>
#include <avr/sleep.h>    // Sleep Modes
#include <avr/power.h>

#define CE_PIN 3
#define CSN_PIN 4
#define LED_PIN 3 //PB3

RF24 radio(CE_PIN, CSN_PIN);


byte address[11] = "1Node";
unsigned long payload = 500;

void setup() {
  radio.begin(); // Start up the radio
  while (!radio.isChipConnected()) {
    digitalWrite(LED_PIN, HIGH);
    delay(100);
    digitalWrite(LED_PIN, LOW);
    delay(100);
  }

  radio.setAutoAck(1); // Ensure autoACK is enabled
  radio.setRetries(15, 15); // Max delay between retries & number of retries
  radio.openWritingPipe(address); // Write to device address 'SimpleNode'
}

void loop() {

  if (radio.isChipConnected()) {
    process();
  }
  if (!radio.isChipConnected()) {
    for (byte i = 0; i < 10; i++) {
      digitalWrite(LED_PIN, HIGH);
      delay(100);
      digitalWrite(LED_PIN, LOW);
      delay(100);
    }
  }

  goToSleep();
}

bool isDelivered = false;
void process() {
  radio.powerUp();

  radio.stopListening(); // First, stop listening so we can talk.
  //isDelivered = radio.write( &payload, sizeof(payload));
  isDelivered = radio.write( &payload, sizeof(unsigned long));
  if (isDelivered) {
    digitalWrite(LED_PIN, HIGH);
    delay(250);
    digitalWrite(LED_PIN, LOW);
    delay(250);
    //updateMessage();
  }
  if (!isDelivered) {
    for (int i = 0; i < 4; i++) {
      digitalWrite(LED_PIN, HIGH);
      delay(250);
      digitalWrite(LED_PIN, LOW);
      delay(250);
    }
  }
}

void goToSleep ()
{
  radio.powerDown();
  set_sleep_mode(SLEEP_MODE_PWR_DOWN);
  ADCSRA = 0;            // turn off ADC
  power_all_disable ();  // power off ADC, Timer 0 and 1, serial interface
  sleep_enable();
  sleep_bod_disable();
  sleep_cpu();
  sleep_disable();
  power_all_enable();    // power everything back on
}  // end of goToSleep