Thermal losses happen 3 ways, conduction, radiation and convection.
Conduction is reduced by insulation.
Insulation is really a combination of a material with a high thermal resistance and small pockets of air (or other gases, and even no gases in the case of vacuum insulation) trapped between whatever material the insulation is made from.
This reduced the movement of the air, which reduces convection.
Air also has a fairly high thermal resistance.
The SI unit is kelvin meters per watt, R = K.m/W.
It is more usual to use the reciprocal 1/R, which has the SI units watts per metre kelvin, λ = W/m.K.
Air has a typical value of 0.0255 W/m.K, polyester has a value of 0.05 W/m.K.
Polyester and paper conducts at about twice the rate of air, 0.05 W/m.K.
Cotton conducts at about ten times the rate of air, 0.23 W/m.K.
Dry earth is even worse 1.5 W/m.K, 60 times worse, and is, in effect, an infinite heat sink.
As you cannot realistically change the ground temperature, or your body temperature, all you can do is increase the thickness of the insulation.
As clothes and sleeping bags compress when laid on, it is therefore best to use an inflatable mattress as this will drastically reduce the losses.