guys seriously this is just yet another reason to write your reporting interval based on battery voltage and time and not on time alone.
You are using extremely cheap systems with cheap batteries of unknown age that are likely very prone to incur reduced capacity over shorter times and memory from discharge cycles.
See the posts above. This really is one of the greatest ideas I have seen for monitoring, just needs a tweak
With these findings I would set a floor voltage around 1-1.1 V for the arduino to go into sleep mode (find your own floor voltage by looking at your sleep discharge rate and length of night time).