Gist of the deep sleep current example from https://medium.com/p/732def7d95c3

deepsleep_nrf52840_seeed.ino

// The MIT License (MIT)
// Copyright (c) 2019 Ha Thach for Adafruit Industries

#include "SdFat.h"
#include "Adafruit_SPIFlash.h"

// Uncomment to run example with custom SPI and SS e.g with FRAM breakout
// #define CUSTOM_CS   A5
// #define CUSTOM_SPI  SPI

#if defined(CUSTOM_CS) && defined(CUSTOM_SPI)
  Adafruit_FlashTransport_SPI flashTransport(CUSTOM_CS, CUSTOM_SPI);

#elif defined(ARDUINO_ARCH_ESP32)
  // ESP32 use same flash device that store code.
  // Therefore there is no need to specify the SPI and SS
  Adafruit_FlashTransport_ESP32 flashTransport;

#else
  // On-board external flash (QSPI or SPI) macros should already
  // defined in your board variant if supported
  // - EXTERNAL_FLASH_USE_QSPI
  // - EXTERNAL_FLASH_USE_CS/EXTERNAL_FLASH_USE_SPI
  #if defined(EXTERNAL_FLASH_USE_QSPI)
    Adafruit_FlashTransport_QSPI flashTransport;

  #elif defined(EXTERNAL_FLASH_USE_SPI)
    Adafruit_FlashTransport_SPI flashTransport(EXTERNAL_FLASH_USE_CS, EXTERNAL_FLASH_USE_SPI);

  #else
    #error No QSPI/SPI flash are defined on your board variant.h !
  #endif
#endif

Adafruit_SPIFlash flash(&flashTransport);


/*  If you want to use a specific flash device, for example for a custom built board, first look for it in Adafruit_SPIFlash\src\flash_devices.h
 *  If it isn't in there you need to create your own definition like the W25Q80DLX_EXAMPLE example below.
 *  These definitions need to be edited to match information on the data sheet of the flash device that you want to use.
 *  If you are not sure what the manufacture ID, memory type and capacity values should be, try running the sketch anyway and look at the serial output
 *  The flash device will report these values to you as a single hexadecimal value (the JDEC ID)
 *  For example, the first device on the list - the W25Q80DLX - will report its JDEC ID as 0xef4014, which is made of these three values:
 *  manufacturer_id = 0xef
 *  memory_type     = 0x40
 *  capacity        = 0x14
 *  With this macro properly defined you can then create an array of device definitions as shown below, this can include any from the list of devices in flash_devices.h, and any you define yourself here
 *  You need to update the variable on line 71 to reflect the number of items in the array
 *  You also need to uncomment line 84 and comment out line 81 so this array will be passed to the flash memory driver. 
 */
//Example of a user defined flash memory device:
//#define W25Q80DLX_EXAMPLE                                                               \
//  {                                                                            \
//    .total_size = (1 << 20), /* 1 MiB */                                       \
//        .start_up_time_us = 5000, .manufacturer_id = 0xef,                     \
//    .memory_type = 0x40, .capacity = 0x14, .max_clock_speed_mhz = 80,         \
//    .quad_enable_bit_mask = 0x02, .has_sector_protection = false,              \
//    .supports_fast_read = true, .supports_qspi = true,                         \
//    .supports_qspi_writes = false, .write_status_register_split = false,       \
//    .single_status_byte = false, .is_fram = false,                             \
//  }

/*
 * Create an array of data structures and fill it with the settings we defined above.
 * We are using two devices, but more can be added if you want.
 */
//static const SPIFlash_Device_t my_flash_devices[] = {
//    W25Q80DLX_EXAMPLE,
//};
/*
 * Specify the number of different devices that are listed in the array we just created. If you add more devices to the array, update this value to match.
 */
//const int flashDevices = 1;


#include <bluefruit.h>
void setup()
{
  flash.begin();
  Bluefruit.begin(); 
  flashTransport.runCommand(0xB9);
  flash.end();

  sd_power_system_off(); 
}

void loop()
{
  // nothing to do
}

How can we wake up it using RTC timer ?
example i took reading from temperature sensor and now put it into deepsleep can i wakeup it again after 1 hour for getting more readings and again sleep? using RTC ?