Thermarest Hyperion 32°F/0°C Review

Thermarest have been in the sleeping bag game for a few years now but they are definitely upping the ante with their latest lines. The Hyperion 32°F/0°C is the type of bag that I find most versatile. It is just about warm enough for summer alpine bivis, when supplemented with a few clothes, but is not too warm for valley camping. It packs into a tiny size so can come along on hut to hut trips or can be stuffed into a kit bag or rucksack without hassle.

What more do you need to know – it meets with the Cat’s approval. Took a good 5- 10 seconds from unpacking to occupation!

Thermarest produce two Hyperion sleeping bags the 32°F/0°C reviewed here and the 20°F/-6°C which is obviously a little warmer and heavier. Talking of weight the Hyperion 32°F/0°C tips the scales at a svelte 454g (460g on the CGR scales) in its regular size (a long version is available). For its given rating the 32°F/0°C is a seriously light bag helped by the 900 fill power Nikwax Hydrophobic down. The specifications include:

  • Quarter zip with baffle
  • Draft collar
  • SynergyLink connectors (to keep a sleeping mat in place)
  • ThermaCapture lining (to reflect radiant body heat)
  • Full box baffled construction (using mesh walls to maximise loft and breathability).

Once stuffed into its special compression stuff sac and compressed you get an extremely petit 0°C bag. The stuff bag is unusually quite long and narrow to facilitate compressing to such a compact size. In fact the narrow theme is reflected in the bag itself as it is noticeably more snug than my other sleeping bags. Being on the skinny end of the body spectrum this is not a problem but you may want to try on/in for size if you are more broad of beam. The use of hydrophobic down (that meets RDS standards) is welcome and although this won’t save you on a wet bivi it does mean that you won’t suffer from loss of loft in damp environments and on multi day climbs/treks when you don’t get the opportunity to air out your bag fully. The down is zoned or as we sometimes call it differential fill with the majority 70% on the top and the remaining 30% on the base – where it is less use and gets compressed anyway. If you twist and roll a lot in your sleep this zoned filling might not work so well for you though. My personal preference with lightweight bags is to have no zip but the quarter length zip does give you easier entry and exit together with some ventilation. However the zip is very light and there is the occasional tendency for it to snag. I’d prefer to have it centrally located rather than on one side for ventilation purposes but this is probably a personal bias and would, I imagine, be more costly to implement. The SynergyLink connectors are a neat idea which allow you to anchor your sleep mat to the bag to help eliminate slippage and are easily removable if not needed. If you are used to sleeping on closed cell mats with an almost ‘tacky’ finish these may seem unnecessary but on many modern ultralite inflatable mats such as Thermarest’s own NeoAir UberLite (review coming soon!) they are a godsend on all but the most level of pitches!
One of the first things that you want to know about a bag is ‘Is it warm enough?’ Once upon a time we had 1,2,3 and 4 season rated bags. The problem was there was no standard as such – one manufacturer’s 2 season could be another’s 3 season. Recently there has been an ISO standard developed ISO 23537 ( which at least give a way to compare bags (at least those with a homogenous filling – fancy sythetic/down hybrids are ‘problematic’). I’ve not read the final version (as it costs CHF118) but a ‘preview’ provides the following for our purposes:

  • Comfort Temperature: lower limit of the comfort range down to which a sleeping bag user with a relaxed posture, such as lying on their back, is globally in thermal equilibrium and just not feeling cold.
  • Limit Temperature: lower limit at which a sleeping bag user with a curled up body posture is globally in thermal equilibrium and just not feeling cold.
  • Extreme Temperature: very low temperature where risk of health damage by hypothermia is possible. (Note – this is a point of danger which can lead to death)(!)

There is still some fuzziness out there as not all manufacturers have adopted the standard – there seem to be some that use the comfort rating rather than the limit rating as their headline figure so it’s worth checking before you dismiss a bag or get over enthusiastic!

In Use
So the Hyperion has a comfort rating of 5°C, a Transition(limit) rating of 0°C and if it gets to -15°C you are in a bit of bother! Less objectively I can say it has kept me comfortable in spring and summer conditions used in the back of the van (waiting to pace runners at ungodly hours of the morning), in various mountain huts and the occasional bivi. Used on successive days in typically damp British conditions I’ve found no loss of loft and I’ve been impressed with how it re-lofts after compression. As I mentioned earlier the cut is more ‘trim’ than many bags but mainly in the leg area so you may want to check the fit if prefer a bit of wiggle room. Having said that I found I could easily sit up and operate a stove etc. Helpfully Thermarest actually publish the dimensions: shoulder 145cm, hip 126cm, foot 109cm these are for the ‘regular’ size. As you would expect of such a light bag the fabric is very lightweight but has so far stood up well. On a rocky bivi I’d be a little concerned about using it uncovered but I always use an ultralight bivi bag anyway (protection plus keeps the sleeping bag clean) so not a major issue. Being so light there is a slight tendancey to snag in the zipper if you are a little industrial with it – there is an anti snag set up but it’s not as effective as some I’ve used. Overall I’ve found it to be a very versatile and reliable sleeping bag for Spring and summer UK use. If you want the ultimate in lightweight 0C bags (especially including a zip) there are very few that can compete. Supplemented with a bit of warm clothing I suspect it will function well on summer alpine bivis with the advantage that it won’t be too warm in the valley – I’ll find out next month!


  • Ultralight
  • Warmth for weight
  • Very compact
  • Damp resistant


  • Narrower legs
  • Lightweight/delicate face fabric (alpine use)

RRP £355


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