When nights are much warmer than expected, you get hot and claustrophobic inside the sleeping bag. Fortunately all but the most exotic will unzip, and under your new-made doona the steam stops coming out of your ears.
Hot nights are rare. The nights when you lie on a cold line underneath, or the sleeping bag zip draws a cold line down your back, or your face starts to sting, are more common.
You can always, of course, buy and pack in a thicker, warmer heavier sleeping bag, but if you don't like down bags on the basis they cost Monopoly money, and will kill you the night they get wet, the extra load on your back will be appreciable. The purpose of this screed is to get as much warmth as you can out of the same sleeping-bag.
Working from the ground up, you can put distance between yourself and the ground, lie directly on closed-cell foam, and use a softer, more comfortable body support that will spread heavy contact points like the hips and shoulders and stop them compressing the sleeping bag filling in these areas. To put in a blatant commercial plug the permaflate is designed specifically to do this.
If you cannot carry a light bag to squeeze inside your main sleeping bag, you can get most of the effect at minimal weight and bulk with a cheap padded nylon 'bib and brace' trouser set as sold to the downhill skiing day tripper. You can wear this round the campfire too. All spare clothing can go underneath your mattress, but if you are on wet snow a light gauge big polythene bag will be needed to keep the clothing dry.
A shirt or towel can be used to cover most of the head if you don't like the mummy-style drawcord round the face. As far as I know, I originated the practice of sleeping upside-down, which in my experience is warmer and considerably more hygenic than either of the above. Some radical sleeping bag designs boast more padding on top, or more padding underneath, but the majority of bags can be turned the other way up. Since under your head is a clothing pillow of some sort, that contact area is already insulated, and all that lovely padded head extension does really is to (excuse me) collect dribble. If you put your head on your pillow with the bag inverted, the padded head extension can go right over your head if things get really chilly, and if the drawcord is not tightened there is no tendency that I have found to cut off the air supply to the mouth and nose. The top of the bag stays unbelievably cleaner, but if you do succeed in suffocating yourself, drop me a line to say how it was done.