We do think the temperature ratings are a little generous, and that to get any sleep at -9°C (15°F) you would need some serious base layers on. This is where synthetic insulation excels and is often the better choice for wet or humid environments, despite being heavier and less packable (in general).

Oex Leviathan EV 900 Down Sleeping Bag | GO Outdoors Oex Leviathan EV 900 Down Sleeping Bag | GO Outdoors

Where other bags use horizontal or vertical baffles to hold insulation, this one uses trapezoids for thermal efficiency and to keep the bag from having cold spots. It has 800 fill down, making it very soft and warm, with a Pertex outer shell that repels the weather. So too do other design features like the carefully considered baffle arrangement, tapered mummy cut, snug hood and contoured foot box.

There are draw cords on both the hood and the shoulder baffle, so things can be tightened up if things get truly Baltic. And do you really need a bag with a comfort rating of -22°C (-8°F) when something half as warm would do? One option I was considering was getting an Alpkit Skyehigh 900 + Hunka XL + Thermarest NeoAir Xtreme + Alpkit Masson sleeping bag liner, would this work?

sleeping bags 2023 | Advnture Best 4-season sleeping bags 2023 | Advnture

We took it on some freezing winter camps in Germany’s Black Forest National Park, we’ve had it out on numerous summits at various times of year throughout the Bannau Brycheiniog and Eryri National Parks. The advantages are that it makes the sleeping bag noticeably warmer, and stops the bag itself from getting smelly and manky. We wouldn't recommend getting into a winter-rated sleeping bag during a July heatwave, but hopefully this is common sense. Conversely, a bag that keeps a cold sleeper comfortable in the winter might make warm sleepers too hot.

In terms of comfort, this is a very lofty sleeping bag with lots of attention paid to keeping unwanted cold air out. Fill power is the most obvious difference - winter sleeping bags often have high powered goose down filling to keep you as warm as possible in challenging conditions. The compression stuff sack lets you pack it down to a very compact and manageable size for packing and carrying.

Oex Leviathan Ev Hydrodown 900 | Sleeping Bag | George Fisher Oex Leviathan Ev Hydrodown 900 | Sleeping Bag | George Fisher

You need fewer feathers the higher the fill power, so down products made with down of a higher fill power (750 or 900) will normally be lighter, more packable and pricier as a result.

I’ve always been impressed by its performance, be that on a rainy thru-hike of Germany’s Black Forest National Park, dozens of winter nights in and around Snowdonia’s Glyderau, or 10 chilly October nights in Wyoming’s Grand Teton National Park. The zip is also lined with a thick strip of protective material, which minimises snags and ensures you don’t accidentally tear the soft inner while zipping the bag up. Derived from the plumage of ducks and geese, it offers incredible warmth for the weight, and packs down smaller than any other insulation on the market. It utilizes superfine yarns bound together with special resins to mimic the properties of natural insulation such as down.