Question
Hi. I live in zone 7b-8a (depending who you ask) in Central Alabama.

I have a few tropical plants that I'm growing in pots.

2 Avocado
1 Papaya
1 Bougainvillea

The past 2 weeks and probably til about October 1st, the temperatures stay between 55 and 65 at nighttime.

My question is, when should I start bringing them into the house at night?

Answer
They can remain outside until night temps begin dipping below 35 degrees or so,,,when frost is possible; Here in Southwest Florida, we experience occasional frost and freeze conditions through the Winter and need to cover our cold sensitive plants with frost blankets. Bring your 4 plants back out during the day so they can enjoy direct Sun, and then move them back in as the temps drop down, that way, you won't need to find an area inside where they will get indirect Sunlight.