Its my understanding that 65-70 is an acceptable night time temperature, although 70ish is prefered.
If you are really paranoid about night time temps (like me!) you can hook your night-time light up to a thermostat, which is set to turn on when the temperature drops bellow a set level.
This is how mine is set up with a MICROCLIMATE MINISTAT 100 (which is pretty cheap). It has a probe inside the viv, and if at night her viv temp drops bellow 70 it turns on, boosts the temperature and flicks off again.
That way you dont disturb your little ones sleep all night long, but means you dont have to sit up worrying at night when it gets super cold.
Just something to consider ofc ^_^