Sulcatas respond to light.
I had my activeUVheat bulb on a timer that came on at 8:30, the sulcata would wake up at 8:45, and hang out under the light until warmed up (9:15ish) and then start being active.
Does he sleep on the hot side?
Mine always slept on my Kane heat mat, and wanted to stay warm all the time.
Operating temperature for these guys is low-mid 90s during the day.
I never let mine get under 80, except for soaking.
The things to watch for are dehydration, impaction, enough belly heat, Ca
ratio of their foods, excessive sugar or protein in their diet, and lots and lots of UVB rays.
Babies should never be allowed to get under 80F, never kept on any substrate that isn't digestible, and must have access to belly heat.
Also take lots of baby pictures, because once they're 25+ pounds, you'll never believe they were ever that small.