Trouble with sleep mode and interrupts Context

I am building a small battery powered counting device. As with many other projects like this, saving power is critical. I am therefore trying to utilise the sleep mode wherever possible. This is using a 328P as a standalone MCU running on the 1MHz internal clock

The Hall Effect sensor I am using outputs Vcc when nothing is detected, and Ground when something is detected. It therefore seems well suited to being used as a trigger for an interrupt to wake up the 328P from sleep (so that I don't even need to use a watchdog timer.)

Below is an excerpt from my code. This code does put the processor into sleep as desired. However, from testing I have discovered that the device will not sleep when the magnet is left near the sensor, as its output stays LOW. As I only want to detect pulses of a moving piece of metal, this is wasting power. I only want to wake up when it is initially detected on each cycle.

void sleepLoop()
{
  Serial.print("S");

  delay(5);   //scales to 40ms

  set_sleep_mode(SLEEP_MODE_PWR_DOWN);
  sleep_enable();
  noInterrupts();

  attachInterrupt(digitalPinToInterrupt(countInterruptPin), sleepInterrupt, LOW);

  interrupts();
  sleep_cpu();

  //The program will continue from here.

  //First thing to do is disable sleep.
  sleep_disable();
}


I therefore changed

attachInterrupt(digitalPinToInterrupt(countInterruptPin), sleepInterrupt, LOW);

to

attachInterrupt(digitalPinToInterrupt(countInterruptPin), sleepInterrupt, FALLING);


PROBLEM

Now my processor never goes to sleep. For debugging I have put in a serial print at the start of the loop so that I know it has entered this loop of code. Whether LOW or FALLING is used the 'S' is printed. I only know it is not sleeping because I am measuring the current flow.

I thought the sensor may be going LOW long enough to keep the device awake, but not long enough to register as a count (as I have a debounce in there) so I have checked the output from the sensor with an oscilloscope in case it was momentarily going LOW, but I couldn't see any sign of this.

The findings from the oscilloscope make me feel like there is something about the interrupt function that I am overlooking. Any help would be greatly appreciated!
Only low level detect is possible with external interrupt pins (INT0/D2 and INT1/D3) in power down sleep mode. The signal must be low for a number of clock cycles to be detected and the interrupt will detected while the signal is low (and the interrupt is enabled). It is not a transition. 

A pin change interrupt could be used instead. 

For more details please see chap. 9.5 Power-down Mode and chap. 12. External Interrupts. 

Cheers!