How long have I been asleep? System.sleep(uint16_t wakeUpPin, uint16_t edgeTriggerMode, long seconds)

Firmware
4

@ScruffR worked perfectly, thank you. I now have this code which works well:

void goIntoLowPowerMode()
{
    int sleepLeft = SLEEP_PERIOD;
    int startSleep = Time.now();
    while (sleepLeft > 0) {
        int rainCounterBeforeSleep = rainCounter;
        System.sleep(RAIN_PIN, FALLING, sleepLeft );
        sleepLeft = SLEEP_PERIOD - (Time.now() - startSleep);
        if (sleepLeft > 0) {
            // RAIN_PIN was tripped because there is time left on the timer
            // Wake just long enough to count the pulse, then go back to sleep for remaining time
            unsigned long timeout = millis();
            while (rainCounterBeforeSleep == rainCounter && millis() - timeout < 10) delay(1); 
        }
    }
}

If the pin falls and causes System.sleep to return, do you know if the ISR also attached to the falling edge of that pin will get called immediately so I can count the pulse, before I get around the loop and back into the sleep? Or is that indeterminate? I’m just wondering if that code I have above waiting for the count to increment is necessary.