A slight drop like that is ok, but it isn't ideal. If the temps drop much more than that you really do need to use a heat lamp to boost the air temps. 75 is commonly agreed upon to be the lowest safe temperature for prolonged exposure. The ideal conditions in a BP cage are:
- 90 F Hot side
- 80 F Cool side
- No night drop unless breeding
- 60% humidity
That's what I shoot for 24/7/365.