ATBs are Nocturnal, So being active at night is perfectly normal.
They are also known to go onto the ground at times (though I have never seen my Girl do this since i've gotten her??)
And this is not really in itself a cause for alarm, however If there always doing it there may be something slightly wrong.
Though usualy with the setup not the snake.

First off, What are the temps and humidity at? and what are you using to test that (dial thermometers? Stick on? Digital probes? tempgun?)

Also, what are the perches like?

ATBs don't like a single branch like a chondro or a ETB, they prefer something more like the crotch of a tree. Right angles, with a bit more space to lay out.
Many even prefer shelves over branches.

Also, Maybe the branch leaves him feeling too exposed during the day.
Alot of ATBs will pick feeling hidden on the ground over exposed on a branch.

I personaly think there should be more then one perch, just like a terrestrial animal should have more then one hide.
And since they mostly stay up off the ground, places to hide among the branches.

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