As potentially one of the bulkiest items in your bikepacking kit, the sleeping bag is one item well worth throwing some cash at in an effort to minimise it's space and weight requirements.
The Thermarest Hyperion 32, as we have discovered over the last few months, is a superb way to reduce that luggage-eating impact. Coming in at 14x15cm when packed in it's handy mini-compression sack, and barely troubling the scales at just short of 500g, it is nothing short of miniscule when packed - especially considering the 0 degree rating.
Being stuffed with Nikwax Hydrophobic down should help allay fears that any dampness will write off the insulation values completely, although that said we have been very careful to keep it all dry in use as we always are with down bags - one damp, thin sleeping bag night years ago has been enough to teach us a lesson, thank you. The 900 fill power was excellent, although the comfort rating of 5 degrees centigrade could be a little optimistic for some, depending on how hot you sleep. With thermal bike gear on we survived comfortably on Scottish autumnal nights down to around 6 or 7 degrees C, but wouldn't describe ourselves as 'cosy' at that temperature. Much colder and we would have been taking down jackets into the bag if it had got much colder. Of course that is a personal thing, but we generally sleep warm overall.
Another casualty of that incredible weight saving was the cut - it was all pretty slim fitting, and if we had opted for a down jacket supplement, it would have to be a thin one or we just wouldn't have fit across the shoulders. The bag is highly shaped to minimise waste, so it might be worth slipping into one before buying to make sure it doesn't feel too claustrophobic. We got on fine with it however, and appreciated that the lack of weight has to show somewhere.
A neat feature was the removable straps for wrapping around a sleeping mat, meaning there is less chance of rolling off in the night and getting chilled that way. If you don't like them, they just slip off and can be left behind. ]
Overall however we loved this bag, with the pack size and weight more than making up for any slight compromises or shortcomings. It is just about passable as a 3 season bag here in the UK, but keep that down jacket handy. Considering that real-world temperature rating of around 6 degrees in our opinion it is expensive at £340 of course, but the fabrics feel reassuringly solid despite being so light, and all the fittings such as the popper at the top of the zip are well thought out. A great addition to any lightweight kit, if you can afford the salty price tag.