Andy,
on more suggestion 
Apart from using the watchdog timer you could also use the light sensor itself to create an interrupt.
I’m no electronics guru, but using an opamp you can measure whether your light intensity goes below a certain threshold, if thats the case you pull a pin of the RFbee high or low and use that to trigger an interrupt. That interrupt will wake the RFbee which means that the RFbee will be fully asleep till dark.
Things to watch out for are of course clouds, raindrops, bird shadows etc which temporarily reduce the amount of light coming in
Cheers,
Hans
ps. as of Monday I’ll be offline for a few weeks due to annual leave…