| Job | Keep you warm sleeping outdoors |
| Rated by | Temperature rating |
| Fill | Down or synthetic |
| Pair with | A sleeping pad (for R-value) |
A sleeping bag is an insulated bag that keeps you warm while sleeping outdoors, rated by the lowest temperature it is designed for. Filled with down or synthetic insulation, it traps body heat; mummy-shaped bags maximize warmth for weight, while rectangular bags trade warmth for room. Pair it with a sleeping pad for insulation from the ground.
Choosing one
Pick a temperature rating with margin, and down vs synthetic fill by conditions; a mummy shape is warmest for weight.
Don’t forget the pad
A sleeping pad (and its R-value) insulates you from the ground. Lighter alternative: a quilt — see sleeping bag vs quilt.
Frequently asked questions
How do sleeping bag temperature ratings work?
Ratings estimate the lowest temperature at which the bag keeps a typical sleeper warm. Standardized EN/ISO ratings give a 'comfort' rating (warmer sleepers/women), a 'limit' rating (warmer sleepers/men), and an 'extreme' survival rating. Choose by the comfort rating and add margin, since people sleep differently.
Down or synthetic sleeping bag?
Down is warmer for its weight, packs smaller, and lasts longer, but loses warmth when wet and costs more; synthetic keeps insulating when damp, dries faster, and is cheaper, but is heavier and bulkier. Down suits dry, weight-conscious trips; synthetic suits wet conditions and budgets.
Sleeping bag or quilt?
A sleeping bag fully surrounds you with a hood and zipper for maximum warmth and draft protection; a quilt drops the crushed-flat underside and often the hood to save weight, attaching to the pad instead. Bags are warmer and cozier; quilts are lighter and favored by ultralight backpackers.
Sources
- Sleep systems — The Mountaineers