I measured the inside temperature in my Nordisk Svalbard 1si on a freezing December night.
Raising funds here for equipment for my scouts unit https://www.buymeacoffee.com/fellmandave

RESULTS BELOW.
Firstly measured it empty; then I left the candle burning inside for 20 min,then measured the temperature again and finally got in the tent for 20 min and measured the temperature again to see the combined effect.
I discuss all the different factors that could influence this experiment and will be helpful to anyone when considering what conditions can cause heat loss in a tent.

Results were:
Temp in degrees centigrade

Outside -0.8c
Empty tent 0.8c
With candle 2.4c (Up 1.6)
Candle + body 8.5c (Up 6.1)

Overall made it easier for me to go from a chilled state to a comfortable one, plus pleasant. The lantern with candle weighs 181g

Part 1 is the one man tent, Part 2 is, you guessed it, a two man tent.
Please be very careful if using any naked flames in a tent, UCO recommend that you do not let the candle out of sight or place it near or near any flammable materials. In other words, don't copy me!

Here is a link to much of the equipment I use and trust. I earn a small commission from using this link at no cost to you, so thanks! https://amzn.to/42MWyvS