Laura: You've asked a
good & reasonable question.
I'm with Guy on this.
The candle claim has been floating around the 'net for decades.
I think the
risk to reward ratio is excessive.
Better to have decent cold weather sleeping gear. It doesn't have to be high-end.
I used to have a very decent cold weather sleeping bag, but lost it in a flood more than a decade ago.
I've never replaced it.
Last winter, I broke down in northern WI/MI right before the Polar Vortex, and spent part of that period sleeping in my van, with just regular blankets, some
mylar sheets/"blankets" (pro tip: put one
underneath you), good wool socks (including an oversize pair as a 2nd layer), good wool watchcap, and lots of layers.
Lowest temp inside my van during that period was -11F.
A candle would not have been a significant improvement, and would have been very risky, even in a nice holder.
It's great that you're thinking about this now!
... cuz... Winter Is Coming.