Have you ever bought a sleeping bag that seemed warm enough on the label, then spent half the night awake and cold? It happens a lot, and it's rarely the bag's fault. It's usually down to picking the wrong rating for the trip.
This is what those numbers on the label actually mean, what else affects how warm you sleep, and which Fjern bag fits which kind of trip.
What comfort, limit and extreme actually mean
Our bags (and most others) are tested against ISO 23537-1:2016, which gives three figures.
- Comfort is roughly the temperature where a "standard woman" sleeps fine, lying relaxed, not cold.
- Limit is where a "standard man" is curled up fighting the cold, not shivering, and should just about make it through 8 hours. This is the bag's real lower boundary, not something to plan around.
- Extreme is a survival number. It's the point where hypothermia becomes a genuine risk. It tells you the absolute floor, nothing more.
People often glance at the lowest number on the box and assume that's what they'll be comfortable at. It isn't. A bag with a Limit of -9°C might have a Comfort rating of -2°C and an Extreme down at -27°C, so there's a big gap between "fine" and "just about surviving."
On our product pages we boil this down to one figure, the Fjern Sleep Limit, based on the Limit rating. Think of it as the coldest you'd want to use that bag in, not the temperature it'll feel cosy at.
Why the label isn't the whole story
Ratings are based on standardised lab conditions, and real camping rarely matches them exactly. A few things that make a real difference to how warm you actually sleep:
- What's under you. The ground pulls heat out of your body far quicker than the air around you does. A decent insulated mat can matter more than the bag itself.
- Eating and drinking before bed. A warm meal and enough fluids genuinely help your body keep itself warm overnight.
- Your own build. Ratings are worked out from standardised models, so body weight, gender, clothing and general fitness all shift things one way or the other.
- How you've pitched your tent. A well-pitched tent blocking wind keeps a pocket of warmer air around you. An exposed pitch loses that.
- Down loft. Down only insulates properly once it's fully fluffed up, so a bag that's been left compressed in its stuff sack for weeks won't perform anywhere near its rating.
None of this makes the ratings pointless, it just means they're a starting point rather than a promise. A safe rule of thumb: buy a bag rated a few degrees colder than the coldest night you expect to actually camp in.
Picking a bag for the season
Summer and fast-and-light trips
When pack weight matters more than raw warmth:| Hygge 120 ULW |
470g, 10°C No hood, stripped right back, built for hut-to-hut trekking and warm-weather bikepacking. |
| Omge 120 |
945g, 9°C Our synthetic option here, and a better bet in humid or damp climates since it keeps insulating even when it's a bit wet. |
| Täcke 200 |
620g 6°C A quilt rather than a full bag, so it relies on your mat for the underside insulation. The lightest way to sleep warm if you're travelling minimal. |
3-season trips
The bag most people should actually own, since it covers the widest range of conditions.| Hygge 400 |
870g, -4°C Handles spring through to autumn without much fuss, and light enough you won't think twice about carrying it. |
| Snarka 240 |
1090g, 2-8°C Synthetic, so it copes with condensation and damp tents better than down, without the extra care that down needs. |
Winter trips
Once frost on the tent is a given rather than a maybe, go up in warmth.| Hygge 600 |
1050g, -9°C Cold-weather warmth without giving up too much on pack weight. |
| Hygge 900 |
1530g, -12°C Built for proper winter wild camping, where warmth matters more than weight. |
Getting more out of the bag you've already got
If your bag isn't quite warm enough, or you want one bag to stretch further across the year, a liner is usually the smarter buy over a whole new sleeping bag.
- Ovn Thermolite adds up to 8°C, made from recycled polyester, and keeps body oils off the inside of your main bag, so you'll wash it less often.
- Ovn Merino+Silk adds 10 to 12°C depending on what it's paired with, and brings merino's natural odour resistance and thermoregulation with it. It also works well on its own in hot climates or hostels.
Both can be used as a standalone summer bag too, so they earn their place in your kit rather than sitting unused most of the year.
The short version
Think about the coldest night you're actually likely to camp on, not the average one. Pick a bag whose Fjern Sleep Limit sits a few degrees below that. If you're torn between two bags, add a liner to the lighter one rather than buying the heavier one you'll rarely need at full warmth.
Get that right and the numbers on the label stop being guesswork.
