If you are looking for a sleeping bag for use in winter, you usually end up with a thick monster that is filled with around 2 kg of down, is advertised by the manufacturer as "suitable down to -30°C" and has the packing size of a fridge (or at least a microwave). Of course, this puts you on the safe side in terms of your own well-being and, in more extreme cases, your wellbeing.
However, the reserves for really low temperatures (especially in our latitudes) are rarely used, and a lot of other material has to be taken along in addition to the sleeping bag, especially on longer trips. In addition, the "real" winter sleeping bags are often simply too warm at temperatures around freezing point.
This is exactly where Mammut comes in with the Sphere UL Winter and offers a sleeping bag that keeps you warm at single-digit sub-zero temperatures without getting too warm at +10° and that with a convincingly small pack size and weight.
I therefore find it ideal for a wide variety of activities, from a bivouac in spring to a multi-day hike in summer to an overnight stay in an unknown winter area. As the sleeping bag weighs in at around 1 kg, it can sometimes even be taken along as an "emergency reserve"
However, you should be aware of what you are getting into with an ultralight sleeping bag! First of all, the tight cut. Turning around IN the sleeping bag? No way, you can only turn around with the sleeping bag. Bend one leg while the other remains stretched out? Not really...
The lack of a thermal collar means that I have to pull the hood down quite a bit when it gets down to -10°C, which makes the sleeping bag feel even tighter. It gets even more interesting when inner shoes and a water bottle have to go into the sleeping bag...
However, the lightweight really does keep me warm down to around -10°C, although you have to bear in mind that there are few temperature reserves. For example, it is hardly possible to wear a down jacket in the sleeping bag and the high-quality down means you should avoid getting the sleeping bag wet or even damp. I spent my only uncomfortable night in it at around -1°C after an unexpected water ingress in the tent... Drying is therefore even more important than with other down sleeping bags. Nevertheless, the sleeping bag is impressive, especially for shorter trips or when the backpack is far too full anyway.
