A sleeping bag liner is a lightweight cloth sack that is usually fitted inside a sleeping bag to provide extra comfort, insulation, and help keep the sleeping bag clean. There are two main types of sleeping bag liners: mummy-shaped liners designed to slip inside your sleeping bag, and rectangular liners (also known as travel sheets) designed for use on their own or inside a rectangular sleeping bag.

Sleeping bag liners have several benefits, including:

• Extra warmth: A sleeping bag liner can add anywhere from 5°F to 15°F of extra warmth to your sleeping bag, depending on the liner material.

• Hygiene: Liners can help keep your sleeping bag clean by preventing dirt and sweat from reaching it. They can be easily removed from the sleeping bag and washed, which not only improves hygiene but also reduces the need to wash the bag itself.

• Versatility: In hot climates, you can use a bag liner or travel sheet by itself and forgo the sleeping bag. Additionally, a liner can be used as a sheet in hostels or budget hotels.

Sleeping bag liners are made of various materials, including silk, cotton, nylon, and polyester. Silk liners are lightweight, soft, and strong, and retain much thermal insulation even when wet. Cotton liners are comfortable and breathable, but not as warm as silk. Fleece-like liners can increase warmth by 10-15°F.

Overall, sleeping bag liners are a lightweight and compact way to improve the hygiene, comfort, and warmth of your sleeping bag.