Therm-a-RestHyperion 20F/-6C Ultralight Down Mummy Sleeping Bag

I tested this in Colorado on three separate nights camping in March - one at 50 degrees, one at 35 degrees, and one at 20 degrees under a Black Diamond beta light tarp (not sealed at bottom edges so you get some wind). I am a hot sleeper, 5'8", 200 lbs, broad shoulders, 38 inch waist. The bag fit well with just the right amount of room for shoulders, hips, and feet. At 50 degrees, I was very comfortable with just the bag. At 35 degrees, I was a bit chilly but was then comfortable with lightweight poly shirt, hat, and socks. At 20 degrees, I was cold with the aforementioned clothes. When I slipped this bag inside an REI 4 season bivy, I was warm at 20 degrees with the clothes mentioned. The primary reason was eliminating the breeze.To preserve the bag and keep it somewhat clean from sweat, I have since purchased a Cocoon CoolMax MummyLiner ($46 on Amazon). The MummyLiner is good for about 10 degrees additional warmth.A challenge for me sleeping in this bag on top of a Therm-a-Rest NeoAir XTherm Sleeping Pad over a Tyvek ground cloth, was that all three were fairly slippery and I kept sliding off of the pad while in the sleeping bag. The REI bivy solved the problem by keeping the bag and pad together, while providing a waterproof liner under the sleeping bag and an eVENT top which lets out humidity (perspiration and water vapor when breathing). The combination of this bag with the sleeping pad and REI bivy is breathable and provides 4 season comfort. Without the bivy, this is a 3 season bag with temperatures above freezing. The 20 degree rating is the rating at which you will not freeze to death and assumes you also are not sleeping on frozen ground.This bag packs up really small (3.5 inch diameter by 6 inch length) in a provided compression stuff sack. For long term storage, Therm-A-Rest provides a larger sack (18 inches x 18 inches) that allows you to keep it in a non-compacted state.

First off: I generally don't write a review on anything until I've broken it. Anyone can tell you about the packaging or their YouTube unpacking, but in order to speak with any sort of authority on a product, you need to have spent enough time with it to have pushed it passed it's limits.The Hyperion has been with me on a half dozen trips now, and half of those were absolute catastrophes.The bag is fine, provided you camp in a laboratory. I do not. Most of what I do is primitive, at best. The type of camping where your ground pad is a bag filled with stripped fir boughs. The times I've used it in a tent, with modern ground insulation, during fair weather, it's been fine... No complaints. But this last weekend I had my third major freeze with it.The first, my lean-to took on snow during a mountain storm, which got the bag slightly damp. It was like sleeping on a sheet of ice. It was also up off the ground, on a cot, so not just regular old negligence.The second time, also in Montana in the Little Belts, we had a drop in temp to around freezing. The bag alone was NOT enough to keep me warm. This was within it's published "comfort range", and I found it to be unacceptable. Keep in mind that I'm not expecting a 5-star resort, here. I just expect to be warm enough to actually sleep.This final time was in the Ozarks. The bag was again dry, but utterly insufficient to stay warm in the 32-40 degree range. I'm floored by this, as I bought this bag anticipating my longstanding hope of finding a bag that was easily portable, lightweight, and sufficiently warm was answered. I've been storing it correctly (in the large storage sack, not the compression sack) when not in use. I've used ground pads, cots, hammocks, and the bare ground itself, and have only been comfortable when inside a tent or rooftop tent.I hate to write this review. If I'm doing something wrong, I'd love to know it so I can get back to believing this bag is what I hoped it could be. But at this point, I feel like I just pitched $450 into a frozen cat hole.

First I want to say, Thermarest as a company is amazing. Their warranty is one of the best and they back all their products 100%. This bag is extremely ultralight with all the bells and whistles you would want for backpacking. The build quality is absolutely exceptional. This bag packs down smaller than any other bag on the market in this temperature range. The only downside I have with this bag is the slender cut. It's a very narrow bag from the neck to the toes. If you are a side sleeper it might not be that comfortable especially if you toss and turn. The pad straps do aid in keeping the bag in place but make it more restricting to a side sleeper. The compression sack is absolutely amazing. If you are looking for an Ultralight hot bag that compresses to a tiny size, get this. Just be aware of the tight cut.

When I received the sleeping bag, the box felt as if it was empty! I'm 5'7" 145lbs. I purchased the small, and it feels just right! The regular would have been too big for me. I haven't had the chance to use it in cold weather, however I'm confident it will keep me alive., if not toasty warm.

I solo hiked in Iceland for ten days with this bag and a small tent. Temperatures got down to around 5-8 deg Celsius, or low 40s. The bag was hot at these temperatures - with a thin wool base layer I’d guess I’d be comfortable around freezing. It’s very small, well built, and light for the warmth.