Most decent sleeping bags these days are tested using an international standard: ISO 23537 (previously EN 13537).

The goal of this standard is simple: make it possible to compare different bags using the same test, instead of every brand guessing their own numbers.

Here’s what that test actually looks like:

1. A heated mannequin with about 15–20 temperature sensors is used instead of a real human.

2. The mannequin is dressed in standard base layers: long-sleeve top, leggings, socks, and often a beanie, similar to what you’d wear to bed on a typical trip.

3. It’s put inside the sleeping bag, which is laid on a standardised sleeping mat.

4. The whole setup goes into a cold chamber.

5. As the air temperature in the chamber drops, the sensors measure how much heat the “body” is losing.

6. Using all that data, the lab calculates a comfort range and several key temperature limits for that bag.

The result is three main numbers you’ll see printed on the label: Comfort, Limit, and Extreme.

Let’s translate the jargon into plain English.

Comfort Rating (T Comfort)

This is the most important number for cold sleepers and most women.

Comfort is the lowest temperature at which an average cold sleeper (often modelled as a “standard woman”) can sleep in a relaxed position without feeling cold.

If you’re someone who tends to feel the cold, or you just really dislike being chilly at night, buy according to Comfort.

Limit Rating (T Limit)

This is the key number for warm sleepers and many men.

Limit is the lowest temperature at which an average warm sleeper (often modelled as a “standard man”) can sleep in a curled-up position without feeling cold.

At this temperature, you’re probably:

• Curled up

• Wearing extra layers

• Not exactly toasty, but not uncomfortable either

If you tend to run warm, or you’re happy to wear thermals, socks, and a beanie to bed, you can usually choose your bag based on the Limit rating.

Extreme Rating (T Extreme)

This is the one that causes the most confusion, and the most bad decisions.

Extreme is a survival rating: the temperature at which an average person should be able to survive six hours without dying of hypothermia, but will likely be very cold and uncomfortable, with a real risk of frostbite or worse.

You should never buy a bag based on the Extreme rating. It’s there to give you an idea of the absolute bottom end of what the bag can handle in an emergency, not what you should aim for on purpose.

Are You a Cold Sleeper or a Warm Sleeper?

The ratings are based on “standard” men and women. But you’re not a dummy in a climate chamber.

Ask yourself:

• Do you often steal the duvet?

• Do you wear socks and a hoodie to bed at home?

• Do you tend to get cold hands and feet?

If yes, you’re probably a cold sleeper.

If you’re the type who kicks the covers off, lies on top of the sleeping bag, or sleeps warm in almost any conditions, you’re more of a warm sleeper.

As a general rule:

• Cold sleepers and most women → choose your bag based on Comfort

• Warm sleepers and many men → choose your bag based on Limit

Why Your Sleeping Bag Rating Is Only Half the Story

Even with ISO testing, that rating is just a starting point. Real-world warmth depends on a bunch of other factors the lab can’t control:

1. Your sleeping mat (R-value)

A sleeping bag only traps heat above you. Most of your heat loss is to the ground, so your mat is doing just as much work as the bag. If your mat has a low R-value (poor insulation), you’ll feel colder than the rating suggests, even in a good bag.

2. Your shelter

• In a tent or hut, you’re protected from wind and some radiant cooling.

• Under a tarp or in the open, wind and clear skies can make the air feel colder than the forecast suggests.

3. Wind and humidity

A 0°C clear, still night feels very different to a 0°C damp, windy, misty one. Humidity and wind can strip heat faster than the lab environment predicts.

4. Your clothing

What you wear to bed matters:

• Dry base layers = warmer

• Wet socks, damp thermals, or sweat = colder

• A beanie in a hoodless bag = surprisingly powerful

5. Food, hydration, and fatigue

• If you go to bed under-fuelled, your body has less energy to burn for warmth.

• If you’re dehydrated or exhausted, your thermoregulation can be worse.

• A hot drink or small snack before bed can genuinely help.

6. Age, body size, and metabolism

Smaller, lighter people often lose heat faster. Older bodies can run cooler. There’s no universal rule—know your own tendencies and lean a bit warmer if in doubt.

So you’re standing in the store or browsing online and still struggling to decide?.. This should help.

Step 1: Know your coldest likely night

Think about where and when you’ll use the bag most:

• Summer camping at low elevation?

• Shoulder-season tramping in the Southern Alps?

• Winter missions above the bushline or snow camping?

Step 2: Decide if you’re buying for “most trips” or “worst-case”

You generally have three strategies:

1. One bag to do most things well

   • Good for 3-season tramping, huts, and occasional colder nights

2. A lighter/summer bag + layers/liner

   • For hut trips, bikepacking, or summer camping

3. A serious cold-weather bag

   • For winter, alpine routes, and cold, exposed camps

Being clear on this up front helps steer you toward the right comfort range (e.g. +5°C, 0°C, -5°C, -10°C, etc.).

Step 3: Add a safety buffer

Once you’ve got a target low temperature, give yourself a buffer:

• Cold sleeper? → Choose a Comfort rating about 5°C lower than the coldest nights you expect.

• Warm sleeper? → Choose a Limit rating about 2–5°C lower than the coldest nights you expect.

Example:

• You expect overnight lows around 0°C on your trips.

• Cold sleeper → look for Comfort around -5°C.

• Warm sleeper → look for Limit around -5°C.

This gives you room for unexpected cold snaps, fatigue, or poor campsite choices.

Step 4: Match your mat and system

Check your sleeping mat’s R-value.

• Summer / mild huts: R 2–3 can be fine

• 3-season tramping: R 3–4+ is safer

• Winter / alpine / snow camping: R 4.5–5+ or a combo of mats

A sleeping bag’s rating assumes a mat with appropriate insulation under you.

Step 5: Adjust for your personal quirks

Finally, layer in your own realities:

• If you always feel cold in huts, skew warmer again.

• If you’re happy sleeping in thermals and a puffer, you can push the rating a bit.

• If you mostly sleep in huts and rarely camp in exposed valleys, you might not need such an aggressive rating.

When in doubt, err on the warmer side. You can always vent a warm bag; you can’t magically add insulation to a cold one.

Right Temperature for Your Adventure

Let’s take all that theory and apply it to the kinds of conditions you’ll actually face in the outdoors, from summer camping trips to alpine adventures.

1. Best temperature rating for summer camping  

• Typical overnight lows: 5–15°C

• Cold sleeper: Comfort around 0 to +5°C

• Warm sleeper: Limit around 0°C

Best option: Illumine Lightweight Down Sleeping Bag

2. Best temperature rating for 3-season hiking trips 

• Typical overnight lows: -5 to +5°C (depending on altitude and weather)

• Cold sleeper: Comfort around -5°C

• Warm sleeper: Limit around -5 to -7°C

Best option: Illumine Plus Lightweight Down Sleeping Bag

3. Winter alpine trips and snow camping

• Typical overnight lows: -5 to -15°C (or colder with windchill)

• Cold sleeper: Comfort around -10 to -15°C

• Warm sleeper: Limit around -10 to -15°C

At this point, you’re stacking a quality mat, solid tent or bivvy, good nutrition, and probably sleeping in layered clothing as part of your system.

Best option: Sierra 650 Lightweight Goose Down Sleeping Bag

Is a -10°C sleeping bag always warm down to -10°C?

Not necessarily. A -10°C Limit bag might keep a warm sleeper okay at -10°C with good layers and a warm mat. A cold sleeper could feel chilly at -5°C in the same bag if their mat is poor, they’re tired, or conditions are damp and windy.

Always check whether the -10°C is Comfort, Limit, or Extreme, and interpret accordingly.

Can I rely on the Extreme rating?

No. Extreme is essentially a “you might survive the night” number, not a performance target. It assumes a very rough experience with a real risk of hypothermia or frostbite. Use it only to understand the absolute bottom end of your bag, not to choose a bag.

Why do some brands show only one number?

Some bags, especially casual camping models or older designs, still show a single generic “comfort” or “recommended” temperature. Without ISO or EN detail, that number is more of a brand guess than a lab result. Where possible, look for bags with proper ISO 23537 / EN 13537 ratings so you’re comparing apples with apples.

How much difference does a liner make?

A decent sleeping bag liner can:

• Add around 2–5°C of warmth (depending on the liner)

• Protect your bag from sweat and body oils so it stays loftier for longer

A liner is a great way to extend a slightly warmer-rated bag into colder conditions, or fine-tune one “do-it-all” bag across different seasons.

Is it better to get a warmer bag “just in case”?

Up to a point, yes, especially if you’re a cold sleeper. But super-warm bags can be:

• Heavier

• Bulkier

• Overkill for summer huts

For most Kiwi trampers, a versatile 3-season bag with a sensible Comfort or Limit rating plus a liner and clothing system is more useful than a huge polar expedition bag that spends most of its life in the wardrobe.

Explore Bushbuck’s down sleeping bags