Most will argue that the bicycle is the most important part of your bikepacking gear list, it is your foundation. I agree, it is extremely important as you spend a significant amount, if not the majority, of your day on the bike. However, when the sun goes down, and you need to hunker down for the night, your bike is no longer on your mind. What really becomes important is that sleeping bag you carried all day long. Everyone is different. We ride in different conditions, react to temperatures in different ways, and we may or may not want to carry that ‘extra weight.’ For me, bikepacking in the summer can be extremely variable as the Western United States is filled with so many diverse climates. I rarely head out for a trip without a sleeping bag. Last year I picked up a Marmot Atom 40. The Atom is one of Marmot’s lightest down sleeping bags, at 1lb 8 oz. The red mummy bag weighs in at just 4 oz above of the Plasma 40 – Marmots high end, lightweight bag which happens to be twice as expensive. The Atom features 850 fill goose down with the Marmot Down Defender technology, a full length zipper, mummy hood with pull cords, and a lightweight ripstop nylon to hold it all together. Temperature Marmot designed this bag for summer bikepackers. Summer is the keyword here. The 40 degree temperature rating that Marmot went with does not coincide with the 50 degree EN comfort rating it was tagged with. Even the lower limits EN rating only reaches 43 degrees. That being said, I used a Sol Escape Bivvy with the Atom all of last summer. While any bivvy will give your bag a lower rating, I think I picked the one with the least effect, so keep that in mind. I found that 60 to 40 degrees was indeed quite comfortable with the lightweight bivvy. Dipping under 40 by a few degrees would force me to use a down jacket or additional layers. When I used this system in mid to low 30’s I was pretty miserable, tossing and turning and waking up shivering. I did camp a few nights in a tent, and one of the nights was at or just below freezing. The tent did wonders, and I was comfortable. While the temperature rating is suitable for my race standards, I could have used more warmth for recreational trips where I am sleeping all evening. Down Defender There are two major issues with down; it is expensive, at least with a higher fill rating, and it does not perform well in wet environments. The Marmot Atom comes with 10 ounces of 850+ Fill Power Goose Down, nearly the best goose down you can get. What separates this bag from the market is the Down Defender. The Down Defender technology is goose down treated for moist conditions. The process takes place while the down is beating treated, down clusters are treated with durable water repellent at a very minuscule level. Marmot claims that this treatment will keep your down drier 10 times longer than untreated down. It is also supposed to dry 30% faster than untreated down, which is supposed to give your bag a longer life. Marmot did all of this while staying environmentally friendly with this being a Bluesign approved textile. So does the Down Defender actually work, or is it just a marketing scheme? Prior to using the Marmot Atom, I had a Big Agnes Zirkle, I think it was from 2010. One thing I noticed the most for the Zirkle was the clumping. After the bag absorbed moisture, it would clump and not distribute the down clusters properly, thus not keeping me warm. With The Atom, I have yet to have that problem, and I have been in a few pouring rain storms where the down needed to fully dry before my next use. Although I have not noticed the down repel water so much, I have noticed that it dries much quicker than your standard down – which could prove to be very important while you’re out in the woods for a few days. Other Features How much thought can one put into a zipper, apparently a lot. The the full length zipper comes with two zipper pulls, one at the top, and one towards the bottom. The lower zipper pull gives you the ability to let the lower part of the bag breath. The zipper also comes with a draft tube sewn inside the bag, extending the length of the zipper. This blocks the cold air from entering, or the warm air to escape. The last zipper feature is a simple piece of cloth sewn in behind the zipper to prevent the dreaded zipper snag, a simple yet important feature when you are getting up to pee late a night. The Atom comes complete with a fully baffled hood and closure cords. One on the top of the hood, and one on the body of the bag, you can cinch them down together, or one at a time. Another unique feature that Marmot uses is their trapezoid foot box. This feature gives your feet more room while keeping the bottom of the bag warmer. Whether or not I noticed the warm from the foot box remains to be seen. The additional space was a big bonus, it widened the bag just enough for my erratic sleep-style and provided me with a comfortable amount of room to move my feet around. The bag is held together with a 20 Denier ripstop nylon exterior and a a 30 Denier interior. The exterior is soft and comfortable for sitting around camp, while the interior never gets sticky and uncomfortable when too hot. While the bag did not perform as I had hoped in some situations, I really pushed it to it’s limits. It was worth the sacrifice in comfort sometimes. One of the most notable benefits of the bag is it’s abilities to be packed. The Atom is extremely compressible. So compressible, that it would easily allow me to pull all my straps on a number of saddle bags completely tight. The bag comes with a provided stuff sack that is even a bit large for the bag itself. Here are a few photos of the Atom stuffed into some bags, next to a 160mm rotor.Overall – between the lightweight fabric, the 850 fill goose down, Down Defender, smart baffles, zipper technology, and compressibility, this bag is perfect for warm nights out on the trail. The height of the regular bag was perfect for me being 5’ 10”. While I would have liked to see more width, I am a erratic sleeper and many people may not see the width as an issue. After going back and forth between the Big Agnes Lost Lake and this bag, I chose the Atom because of the weight savings. Because of that I sacrificed warmth, but for those warm summer nights, with a bit of mist falling, I can feel positive that I will remain warm and dry. The Atom retails for $299.99 in regular and long version at 319.99, both bags come in at 1lb 8oz. Check Marmots website for local retailers near you.