Hello. if your monitor is healthy, which is always the first question, then you can look elsewhere for reasons for this inactivity. You say it it, good.
This time of year, your question is common all over reptile boards. It happens with snakes and lizards, it happens to snakes and lizards that are suppose to hibernate(poor term) and species like yours that are not suppose to hibernate. To its really not about hibernation is it?
This time of year, temps outside are dropping and this effects the mass temps, that is, the floor, the walls, the shelves, all these objects are not radiating cool temps, instead of warm temps. The cage, the substrate, the logs, etcs, are all cooler then they were in the summer, even if the air temps are the same. You must understand, or try to understand, reptiles do not use air temps as a guide to activity, they use mass or substrate temps to gauge their activity. In nature, the sun heats up the mass. Not the air temps. The mass or ground or rock or tree truck is a heat sink for temps, absorbed from the sun, not absorbed from the air.
What this means is, while your air temps are the same as it was, the mass temps do not have to be, they can now be lots cooler or a little cooler, depending on what part of the country you live it.
Then you can consider, a little lowering and related inactivity is not a problem for short periods, and decide to not worry or change it. Or you can compensate and increase the usable temps.
I cannot tell you how to do this as many folks approach this in different ways. Some use high heat bulbs and have them a long distance away from the lizards, this heats the air, but not really the lizard or mass. I choose to different approach, I use small bulbs that the lizards can get very close to, even touch. This works great for me.
I recomend getting an infrared heat gun, as advertised above, and use it, they measure mass temps and not air temps, so now you are on the same page as your captive.
It could also be lack of humidity, in winter and use of heaters in houses, this causes humidity to drop. Give that a check if all the temps are close.
Heres a pic of a heat gun in use,
Heres a pic of a small lite, heat source, with the monitors getting very close. As evidence of success, these baby ackies have grown from 7 inches at hatching, to 11 inches in 7 weeks. I guess it could be better, but its still solid growth. I think the growth is now about me feeding them, then about temps.
Good luck FR
