hi jam, it might be feeling the effect of the seasons change and wanting to go into brumation - short hibernation they do.
i currently run grass carpet with ours not sand, but others run sand with no problems
if your concerned bring general temp of enclosure up a bit as he shouldn't be too cold to the touch
what is nighttime temp and how long are your lights on for, what type are they ?
there are some really good previous threads in the reptile section on here that cover beardie keeping
go through those they will probably help as some of the posts are by some really experienced keepers