With a top quality down fill and well-considered design, the new Mythic boasts the best warmth-to-weight ratio of any of Rab's sleeping bags. The range runs to three models: the 200 for summer conditions; the 400 for 3-4 season use; and the 600 for winter cold. With weight-saving warmer weather use in mind, I went for the Mythic 200, at just 482g. And the verdict? This is an absolutely stonking ultralight sleeping bag. For UK summer and bridge season use I think the Mythic hits a magic sweet spot between weight and warmth.

Fill and warmth

Rab have gone all out for quality with the Mythic 200, stuffing it with 200g of 900 fill power ethically sourced European goose down. This should mean maximum loft - and hence warmth - for minimum weight, and the Mythic certainly feels very snuggly for its featherlight weight and small pack size. The loft is excellent; it packs down well but puffs up nicely once unpacked to give you a bag that looks like it has rather more than just 200g of fluff inside.

Every metabolism is of course different, and so too is every situation in which you use a sleeping bag. The variables include absolute temperature, wind, humidity, your state of tiredness, how well fed you are, how vented your tent is, and the quality of your sleeping mat. For this reason the temperature ratings provided by official testing should only be taken as a rough guide no matter how accurate they may have been in lab conditions. According to EN ISO 23537-1:2016 standard testing, the Mythic 200 has a 'comfort' rating of 6°and a 'limit' of 1° (we can ignore the 'extreme' limit of -13° since no one wants to flirt with hypothermia). I tend to sleep warm, and for me the manufacturer's own 'Rab Limit' of 1° feels bang on; unlike the lab figures this is a more anecdotal guide derived from real world use, and my experience bears it out.

As is often the case with a lighter bag, it's easy to expect too much of the Mythic 200, encouraged, I think, by its high quality feel. I've found it a bit of a gamble in the early season Scottish mountains, for instance, when temperatures below freezing can't be ruled out. On two separate trips back in the spring, both very windy, night time temps were predicted to be a couple of degrees above zero at the height I was planning to camp. But of course forecasts are just that, not firm promises. On the first occasion it seemed to stay the positive side of zero all night (the snow in my porch remained soft); I wore a base layer top and bottoms inside the bag and felt comfy throughout. On the second trip the temperature nudged a bit lower than expected (as evidenced by condensation forming as frost on the fly) and I shivered at night until I thought to add a duvet jacket.

On both occasions I paired the Mythic 200 with a mat that was easily warm enough for the conditions, so none of the cold I felt was from below. The fault, of course, was mine and not the Mythic 200's. Use it in the temperature range for which it was actually designed, chilly but above freezing, and I think it'd be hard to beat the insulating performance at this weight.

Hydrophobic treatment

The down has been given a Nikwax hydrophobic treatment which helps it resist moisture uptake, keeping it light and fluffy in damp conditions for a lot longer than untreated down. Nikwax claim that their treated down absorbs 90% less water and dries three times faster than equivalent untreated down, and that it also maintains its warmth-trapping loft a lot more effectively.

Before hydrophobic down was available I was always wary of using a down sleeping bag in damp weather, and often chose a synthetic bag for fear of getting my down one wet. But after recent experience of hydrophobic-treated down I have changed my tune. This stuff is a lot more viable for the British climate. Having slept out in the Mythic 200 in both rainy and humid weather - including a bivvy in the Cuillin during which it rained for several hours - I have yet to notice any clumping up or loss of loft.

Weight and packability

At just 482g (Rab say 475g), the Mythic 200 counts as ultralight in my book. For a sleeping bag that really is impressively minimal, even when you add 33g for the stuff sack, and it packs down pretty small too. With taped seams and a rolltop closure its robust drybag-style stuff sack is all you need to protect and compress the sleeping bag (no fiddly straps here). Once everything is scrunched down it's as compact as many inflatable camping mats, and lighter than some. All this makes the Mythic 200 a brilliant choice for weight-and-space critical summer activities such as backpacking, mountaineering or bikepacking. For general travel in temperate regions it'd be spot on too.

You can get lighter bags that aren't as warm, and warmer bags that aren't quite as light, but there aren't many that compare directly with the Mythic 200 on both counts. After a quick scan of what's available, the closest I could find was the Lightwave Firelight 250 (500g total weight, 250g of 900 fp down, 1C, £449).

Fabric

Pertex Quantum fabric has been used both inside and out. At only 7 denier this is extremely light stuff, and this lightness allows the down fill inside to loft fully. Its tight weave gives the fabric a good level of wind resistance - something I can vouch for, having sat outside in the bag on a cold breezy spring morning - while a DWR finish helps it shed moisture. Though it feels super-thin the fabric also seems relatively tough (and, crucially, down-proof), with a mini ripstop pattern for extra durability. Pertex Quantum is highly breathable, which is of course what you want in a sleeping bag. It does however feel a bit clingy on bare skin; I guess you can't have it all.

Size and fit

The classic mummy shape saves weight, and keeps the bag close all round for maximum efficiency. But Rab have still kept the fit reasonably generous, so you can wriggle about a bit and don't feel too straitjacketed inside. I am 183cm-ish tall, and relatively broad, and on me the Mythic is pretty much a perfect fit; if you were much larger then you'd want to try before you buy. The official dimensions are: length 185cm/72.8in; shoulder width: 70cm/27.5in; hip width: 52cm/20.5in; foot width: 41cm/16.0in. As someone with ridiculous size 47 feet I love the angled footbox, which has plenty of toe space even for me.

Construction

Rab say the Mythic has a 'weight saving baffle construction', which prevents down migrating around. The baffles are a trapezoidal shape, apparently, and angled in diagonal chevrons to keep the down 'over the centre of the body to ensure core warmth throughout the night'. There's also a 'proportionally assigned differential cut'. I'm no expert on sleeping bag construction, so I'll just say that it looks smart and seems to work as advertised.

Extra features

To save weight the zip is only 1/4 length, which is just enough to aid you getting in and out of the bag or to free up your arms for cooking or dressing. In colder weather a 1/4 zip is fine, but once things get warmer it can be nice to vent your sleeping bag right down to the feet, and a couple of times so far this summer I've missed having a full-length zip. If offered the choice I'd probably have opted for a slightly heavier bag that can be opened right up.

On the plus size the zip has a double zipper that offers some ventilation without having to undo the hood, a nice fluffy internal baffle to keep draughts out, and a glow-in-the-dark zip pull.

Inside you get a snug anti-draught neck collar, and the drawstrings on this and on the hood have nice big toggles that are easy to use. The hood, meanwhile, fits snugly around the head and face.

Summary

Premium down sleeping bags don't come cheap, so the Mythic 200's £360 price tag definitely represents value for money. Its high quality fill, lightweight fabric and general build all contribute to the impressive warmth-to-weight ratio and small pack size, while the use of hydrophobic down boosts its damp weather performance. If you're after the lightest possible sleeping bag for weight-critical activities in just-above-zero temperatures, then it would take some beating.