Sleeping bags: how to read comfort, limit and extreme temperatures (and not get cold)

Few things ruin a trek like a sleepless night spent shivering. And often the problem is not that the bag is bad, but that it was chosen badly or its label was misread. Let us sort it out.

The three temperatures on the label

The ISO 23537 standard, which replaced the old EN 13537 in 2016, sets a common laboratory method for measuring a bag’s insulation. On the label you will find:

Temperature	What it means
Comfort (T-comf)	The temperature at which a standard person (female reference) sleeps comfortably in a relaxed position
Limit (T-lim)	The temperature at which a standard person (male reference) sleeps curled up without excessive cold
Extreme (T-ext)	A survival temperature: risk of hypothermia. It is not a temperature to sleep at

Choose by the comfort temperature. The extreme one is a safety figure, not a usage one.

These figures are measured with a mannequin, standard underwear and a reference mat. If you get cold easily, add a few degrees of margin.

Down or synthetic

Down

• Pros: the best insulation for its weight, very compressible and durable if looked after.
• Cons: loses insulating power when wet; more expensive.
• Fill power: indicates the down’s ability to expand. The higher it is (for example 600, 750, 850), the more it insulates per gram. The down/feather ratio (90/10, for example) also matters.

Synthetic fill

• Pros: insulates even when damp, cheaper, easy to wash.
• Cons: heavier and bulkier for the same temperature; loses loft over the years.

Which should I choose?

• Dry cold and weight critical (high mountains, long treks): down.
• Humid climates, heavy use or a tight budget: synthetic.
• Summer and huts: a light bag or a sleeping bag liner may be enough.

At the end of the article you will find a comparison by product type.

The mat: half of the system

The ground steals heat by conduction, and the bag, squashed under your weight, insulates very little underneath. A mat’s insulating ability is expressed as its R-value, measured since 2020 with the ASTM F3340 standard:

• R 1–2: summer.
• R 3–4: three seasons.
• R 5 or more: winter and snow.

You can combine two mats and add their R-values together.

Tricks to sleep warmer

1. Eat something with fat and carbohydrates before going to sleep.
2. Get into your bag warm, not shivering: walk around a little first.
3. Wear dry clothes to sleep, plus a hat and socks.
4. A liner adds a few degrees and protects the bag.
5. Do not cover your face with the bag: the moisture in your breath condenses inside.
6. Ventilate the tent to reduce condensation.

Caring for your bag

• Store it loose, not compressed, in a large storage sack.
• Wash it rarely and with specific products (especially if it is down).
• Dry it thoroughly before storing it.

If you are preparing your first night out, review the gear list for sleeping in the mountains. And if the sky is clear, it might be a good night to look for the Milky Way.