By integrating the sleeping bag and sleeping pad into a single, cohesive unit, Big Agnes is able to offer a more spacious, more stable, more comfortable sleep in a smaller, lighter package. We tested it on a five-day backpacking trip to the Lost Coast over Christmas.

The system pictured here is composed of the Big Agnes Mystic SL 15 bag and Q-Core pad. I'm 6'2", so both are size Long.

What's It Supposed To Do? When you lay inside a sleeping bag, you compress the insulation underneath you, minimizing its ability to keep you warm. So, Big Agnes simply eliminates the bag's insulation in that area then pairs an insulated, full-length sleeping pad to add that warmth back in.

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This particular system takes advantage of that weight and space saving to maximize interior volume with a semi-rectangular shape. As the name would suggest, the Mystic + Q-Core combo should be comfortable to well below freezing. That's with an all-up weight you can get as low as 3.5 lbs (regular length, SL pad). Not the lightest option on the market, but that's a relatively light weight for a sleep system intended to maximize comfort in wet and cold weather.

How's It Supposed To Do It? There's many sleeping bags or even single-layer quilts out there now that ditch the bottom insulation. What makes Big Agnes' solution unique is that they've purpose-designed their bags and pads to work together, achieving an optimal integration that still evades other manufacturers.

The bag is designed to wrap around the pad all the way to the ground, providing insulation over two-thirds of your circumference and the insulation within it is packaged in specifically tailored densities to maximize warmth where you need it and cut weight where you don't. Big Agnes uses a proprietary system to control the movement of the down within the bag; interior baffles keep the insulation in place while allowing your body heat to distribute uniformly. So, there shouldn't be cold spots around your legs or feet.

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The Mystic is a full-featured bag, meaning there's a sturdy, full-length YKK #5 zipper that's accessible both inside and out, a significant zipper baffle and clever features like an integrated pillow sleeve and protruding face baffles that can more fully seal off that area when you've got the bag cinched tight.

Additionally, the Mystic is purpose-designed to be zip-together compatible, easily enabling you to connect the bag to your girlfriend's. Just make sure one of you orders a left zip and the other a right. Thanks to the sturdy full length zipper and the way the insulation continues down the side of the pad, below it, this should lead to as cohesive and draft-free a solution as is possible with two separate pads and bags.

The Mystic uses 800-fill DownTek insulation. A down's fill power is its ability to fill cubic inches with a single ounce of feathers. So, in this case, the down used in the Mystic can fill 800 cubic inches with a single ounce. That loft is what provides the insulation by trapping air and the high fill power is part of what leads to the Mystic's low weight (for a full-featured, semi-rectangular bag), in addition to its light-but-strong ripstop nylon construction.

Both the Q-Core and lighter Q-Core SL pads designed to work with the Mystic and other BA bags use high-quality primaloft insulation in a thickness appropriate to match the bag's 15-degree rating.

Drying out some seriously wet gear. Thank you, sun. You can clearly see the spacious, rectangular footbox here, as well as the generous loft of the down.

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How Does It Perform? The headline gives this one away, huh? The secret here is fully integrating the rectangular bag and pad. Doing so creates a one-piece structure that you sleep inside, not partially in and partially on, as with a non-integrated bag and pad.

In our big sleeping pad comparison, we (as I now realize) unfairly criticized Big Agnes' pads for being slippery and didn't like the larger side baffles that cradle you towards the pads center. But, both criticisms were only applicable when used with a traditional, separate sleeping bag. The pads are actually designed for the integration you see here and deliver comfort, support and stability in unprecedented luxury when used thusly. I still prefer the degree of firmness to which you can inflate any of the Therm-A-Rest NeoAir range (many of which pack smaller than either Q-Core), but Big Agnes more than makes up for that with the degree of integration in its system. As noted in that comparison, the 4-inch thick Q-Core inflates with just 20 breathes and its twist-close valve is easy and convenient to use, easily allowing you to block it with your tongue between breathes. Deflation is more complicated, requiring a few rolls to get all the air out before packing down to its minimum volume.

Where, inside a traditional mummy bag on top of a pad, the bag rolls with you as you move around throughout the night, inside the BA system you're free to toss and turn as much as you'd like while the integrated bag and pad remain absolutely still. That's an unprecedented freedom of movement that comes without sliding off the pad or getting bound up in a twisted bag. That, in this Mystic + Q-Core system, you also get the spacious foot box of a rectangle is an added bonus.

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The integrated pillow sleeve is another luxury. I typically stuff my clothes into the sleeping bag's stuff sack, then lay that either inside a mummy's hood or underneath it, depending on space. Here, you can put that stuff sack in the sleeve, which prevents that from moving too. For you sissies, BA sells an inflatable pillow specifically sized to work with this sleeve. The mummy hood here is spacious enough to get a hand up under the pillow with ease.

On the Lost Coast trip, we slept in temperatures hovering just above freezing, but not below it. That's not a full test of the bag's 15-degree rating, but I can at least report that the thing was almost too warm at 34-ish degrees. I never needed to zip it fully closed or cinch the hood tight.

As you'd expect from a winter trip to the Lost Coast, we got wet. It rained so hard that water invaded my tent and I only had the chance to dry out the bag on two of the days. For four days, it rode inside my pack alongside a sodden tent and other wet gear. Despite getting wet, the down insulation retained its ability to loft and therefore provide warmth. Thank the DownTek coating on the feathers for that; that technology has proved equally effective on other bags we've tested too.

Both the Mystic bag and Q-Core pad are of extremely high quality and very robust, despite their light weight. With a half-wild, 85 lbs dog running amok inside my tent, my camping amounts to abuse testing of any gear that's in there with him and we had no problems with either item. The quality is tangible and I have no doubt both items will last many trips and many years of hard use.

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The reason the non-SL Q-Core is tested here was just that I asked for it at the last minute from BA and they didn't have the SL in-stock in the long/rectangle size. You have to pair the bag length and pad length, as well as the shape, for the system to work right and we only had a regular Q-Core SL in mummy leftover from that comparison test. However, having spent time sleeping on both, I can report that while the SL weighs 10 oz less, it's just as comfortable and just as robust. We'd recommend paying the $20 premium for the lighter SL.

Now that's a zipper. Full-length, snag-free, accessible from both inside the bag and out, robust and backed with a hefty baffle.

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How Does It Compare To Rivals? There's other bags vying for new levels of comfort in backcountry sleep. None of them work nearly as well.

Sierra Design's Back Country Bed is a notable example with which I have much experience. I like it, but I love the Big Agnes. The equivalent Back Country Bed (800-fill, 3-season) is slightly heavier and its pad sleeve is only half-length. Its foot box is also noticably tighter and it doesn't pack nearly as small as the Mystic, which I managed to squeeze into a 5-liter Sealline waterproof stuff sack. It's larger, heavier, doesn't pack as small or sleep as comfortably, but the Backcountry Bed is about $60 cheaper.

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You could use other 20-inch wide, equivalent length, rectangular pads with a Big Agnes bag, but doing so wouldn't achieve the level of integration possible with an all-BA system.

At $470-ish for the bag and $140-220 for the pad (both priced dependent on size, pad price on size and spec), this is not a cheap solution, but it is the most comfortable available in a cold weather sleep system.

It's difficult to illustrate how well the pad fits the sleeve or how fully the insulation encloses its sides and ends. But you can at least see the slot through which you can pull the pad down to the bag's bottom end.

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Adventure Ready? Well, I'm going to be carrying and sleeping in this over anything else available for the foreseeable future. The system is reasonably light, and packs very small, particularly given its maximal comfort. A good night's sleep is incredibly important if you're doing difficult, dangerous, exhausting stuff in the outdoors and you won't get a better one than in this Big Agnes Mystic SL + Q-Core combo.

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