Interesting, indirect I had same question in this thread, only not regarding UVB light and just the lighting cycle.
https://www.chameleonforums.com/threads/skin-can-tell-if-its-day-or-night.182625/#post-1668821
Probably my question can be answered from field experience, pure out of curiosity.
This is funny, I´ve been branded to ask a question regarding this matter.
Because I´ve noticed they keep their 12h rhythm, also during the summer. My cham lives and sleeps in our very bright living room (free-range), with natural (sun)light coming in by sunrise and leaves with sunset. Meaning that it already stays light for more then 2 hours after her lights go out at 07:00pm, still she will go to sleep within 30 till 45 minutes after the lights go out, while it´s still very bright in the living room.
And I´m really, really curious if they behave the same in the wild or if they keep awake till after sunset?