Moisture Wicking Sleeping Bag Liner
Layers are superior for climate control. It is easy to remove a layer when it gets too hot and just as easy to add a layer for extra warmth. Bear in mind too, it is much easier to launder a liner, than it is a thick sleeping bag.
This Moisture Wicking Sleeping Bag Liner is great for sweat control, it is an excellent first layer for skin contact.
In cooler weather we use a Microfleece sleeping bag liner, which also doubles as a warm blanket by itself.