Nothing beats a nice warm down sleeping bag, but if all you have is a summer sleeping bag and you cant afford to go out and get a good winter sleeping bag, a fleece sleeping bag liner is a great second option. I have a cotton / silk liner that I use sometimes during the summer for my own sleeping bag, more because I don't like the feeling of nylon when sleeping. But even a cotton liner adds a couple degrees of warmth.
A fleece sleeping bag liner or a liner made out of any fabric does help to keep the inside of your sleeping bag clean too. It's not the most lightweight option, but it really does help, and some liner materials, like silk, are extremely light.
Fleece Sleeping Bag LinersHere are a couple of examples of fleece sleeping bag liners.
- Kelty Fleece Liner - This is a rectangular sleeping bag liner, which would, obviously fit a rectangular sleeping bag. It has a drawstring closure on top, which is a good feature to look out for because you can draw it closed around your neck and keep the warmth in. It doesn't have a hood, however, and unless you don't mind wearing a toque to bed, a hood is a good feature to have. It adds about 10 to 15 degrees of warmth.
- Sea to Summit Toaster Microfleece- This is a mummy shaped fleece sleeping bag liner from Sea to Summit. This one comes with a drawstring hood as well as a hood, so a better option than the Kelty liner. It doesn't state temperature boost range, but it's probably around 10 to 15 degrees. Weight is 560 grams.
- Sea to Summit Reactor Thermolite Liner - This is not a fleece liner, it's made out of Thermolite. It does, however, add 15 degrees to your sleeping bag and weighs just 230 grams. So half the weight of the Toaster Fleece above, but roughly the same temperature boost.
- Cocoon Coolmax Mummy Liner - Coolmax is another alternative to fleece. It's a popular material for a lot of backpacking and hiking clothing. The liner weighs 255 grams and adds 8 degrees of warmth, so it's not comparable to Thermolite or fleece. That said, I believe Coolmax would be a more breathable material, and might be a better choice for someone looking for breathability.
Of course the sweater doesn't have to be fleece, in fact a down sweater / jacket would probably be a better option. Down is what most good, lightweight sleeping bags are made out of, after all, and you will get more warmth for less weight with a down garment.
That highlights one thing that you should always be looking for, which is dual-use items. A sleeping bag liner is only good for lining your sleeping bag, but a sweater and pants combo work to keep you warm in your sleeping bag and around camp too.