It sounds as though that may be what he is doing. This is the season brumation occurs, in the fall or winter.
Your collared will become less active and eat less (or stop eating all together!) Lay around and be inactive, and get you very worried :0)
I would keep an eye on him to make SURE hes okay, If there are no other signs of illness, he may just want to sleep.
Before I would attempt to fully brumate him I would just watch him a few more days to see how he is.
BRUMATION
To brumate, people will let them go for about 7 days with no food. Then turn off the basking light.
In about 10 days after the basking light has been turned off, you can then turn off the UVB light.
I do not know how cold it gets there, I believe quite cold. I leave my guys at room temp, but most people have the temps cooler at about 55. You can allow them to take a rest for anywhere from 4 weeks to a few months.
IMPORTANT **** At any time if your lizard appears to be losing weight while in brumation, STOP!
And do not brumate any longer.
BUT if all is fine, you have to continue to check on them and give them some water a couple times a week, so they do not dehydrate.
When ever you decide the nap is over, if it were 4 weeks or whatever, you them REVERSE the steps, you did before the sleep.
First you turn the UVB light back on, then after a while turn the basking light back on(the heat) and then when he is fully awake and heated you begin to feed him again.
Laurent, I sure hope thats all it is. I have a few that have not eaten in a week or more also. But I make sure they are fine before I brumate also.
Let us know how he is !
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Eve / PHEve