But a sauna involves a CHANGE in temperature both when the temperature is raised (causing your body to sweat in an attempt to lose heat) and then change when you have a cold shower and shiver to raise body temperature again.

At night (if you turn the central heating off and leave the window open) you body temperature will drop. If the temperature change is too great you may shiver and that may wake you up but brown (or perhaps even biege) fat tissue can (if you do not have dysfunctional mitochondria) upregulate their heat generation potential and keep your body warm (or sufficiently warm to stop you shivering and waking up) automatically.

But the ability of brown/beige fat to generate heat as required depends on how much of this brown fat you have and that depends on your adiponectin levels Keeping omega 3, vitamin D and magnesium levels optimum will raise adiponectin.

That is probably what is going on with Bitter Melon and thermogenisis and Green Tea and Caffiene