Ideally, everyone should be using some sort of sleeping bag liner: it protects the bag and means you won't have to wash it as often (which eventually degrades the down or synthetic fibre); but if you need more warmth than protection, this is the liner to choose. Fast-drying, premium quality Thermolite yarn is cuddly and soft but lighter and more packable than fleece.
The Thermolite Reactor Compact Plus increases sleeping bag performance by up to 11C with its dual weight Thermolite fabric. Specific panelling of 110g/m_ Thermolite in core body and foot bed areas retains heat, while the ergonomic design shaves off excess weight for a snug fit. Ideal for gram counters under 6ft tall.