GBA #4: Can't Sleep (Part 1) pg. 3
Something that I haven't mentioned in the comic up until this point - but it comes up in abundance in non-comic art - is that Porygon-Z can remove their heads. Check out this mildly-NSFW-because-of-a-naked-butt lore post about the species for more details.
Sandra's fire hair - and Chandelure in general - gets lesser depending on how tired they are. When asleep, their fire extinguishes completely. Their scalps are mostly toasty-warm and glow a little bit.