Good one too Ross, must admit to not being adverse to using some sort of hot water bottle when sleeping out, either a mini rubber one or a Nalgene (which will take the heat). Makes a huge difference on a cold night and during the day you have water to drink. (Also means in bad conditions you don't have to melt snow for breakfast so saving time on those "Alpine" starts.) I'm sure like you say that our forbears wouldn't have been against stacking the odd for themselves. whether a fire or sleeping on an ember bed. Keeping your sleep system clean also helps with loft - used to be appalled at the state of some sleeping bags coming in to be cleaned. You wouldn't let a dogs bed get in that state.