good question!
I'm not all that sure, but from what I have read on the forum, the majority of axies seem to become (and are more) active in the evening and at night, as such mine are no different. this is also why I choose to feed mine at night before I go to bed (usually around 9-930ish I start work early!), as they can be seen trotting around the tank at that time looking for food.
when I wake in the morning (about 4.30am) they are chilled and stationary and seem to remain "dormant" that way for most of the morning unless they are disturbed. but come late afternoon, they seem awaken and the process starts again.
I think it probably has something to do with the fact that I only keep the light on the tank switched on when I'm at home (after 3pm till my bedtime)