Review: Ultralight Meets Ultra-Durable in the Sea to Summit Spark SP II Sleeping Bag

I was absolutely shocked by how lightweight and compact this sleeping bag is!

Sea to Summit offers a full line of ultra-lightweight backpacking gear that works well for a bikepacking. The Spark SP II Sleeping Bag pairs nicely with the UltraLight Mat.



The Spark SP II (subsequently referred to as the “Spark”) is an ultra-lightweight sleeping bag designed for traveling fast and still surviving cool temps at night. The Spark compresses down to just 2.9 liters and weighs a scant 16.4oz/464g. The total fill weight is 9.9oz/280g. The lower temperature limit of this Spark model is 35F, with the extreme temperature limit being 9F. However, notably the recommended comfort temperature is much higher: 44F.

The Spark SP series has two other bags with different temperature ratings and weights–the I with a 46F rating and a 12.3oz weight, and the III with a 25F rating and 22oz weight. I chose to strike the balance between them with the Spark II.

Additional specs:

  • Shell: 10D UL Nylon treated with DWR
  • Liner: 15D nylon soft touch, high density weave
  • 850+ loft Ultra-Dry Down® (90/10 European goose down)
  • 1/3 length #3 YKK zipper
  • Sew-through transverse baffle construction
  • Includes lightweight Ultra-Sil® compression bag, mesh storage cell and laundry bag

Out on the Trail


I must confess: I’m a bit of a noob when it comes to ultralighting, and I was absolutely shocked by how lightweight and compact the Spark sleeping bag is! Sure I’ve used other backpacking sleeping bags before, but the Spark weighs just a pound and compresses way, way down–both of major benefit while packing frame bags.

Sleeping bag in compression sack. Banana for scale.
Sleeping bag in compression sack. Banana for scale.

I also found the Spark to be quite comfy. The real down filling was soft and comfortable, and the bag wrapped around my body securely.

Some people prefer a wider bag so that you can roll over inside of your sleeping bag, but if you’re truly trying to ultralight and pack as light as possible, those little creature comforts just end up adding weight. While I wasn’t able to roll over in the Spark bag, it was comfortable enough in all directions that I just found myself rolling the entire bag when I had to move in the night–and being absolutely OK with that.


While light weight and packability are somewhat par for the course for an ultralight bag, I found myself seriously impressed with the quality of the construction of the Spark. With some ultralight products I find myself nervous about destroying them on accident, but the zipper on the Spark pulls confidently from all angles, and I never felt like I was going to rip it out of the seams. The exterior of the bag felt quite light, yet I also felt confident living in it and even lounging on it when not sleeping.

Three short trips with the Spark isn’t enough to truly assess the long-term durability of this bag, but based on the high quality of the construction I’d feel confident packing the Spark for a lengthy journey.



While I love the light weight, the packability, and the quality of the construction of the Spark SP II, I never managed to have a warm night in the backcountry in this sleeping bag. When choosing the bag I consulted with an experienced backpacking friend, and we settled on the II as a compromise between light weight and warmth. However, based on the spec list we did not realize that the 35F number wasn’t the comfort level but rather, a minimum.

While the Spark II would undoubtedly be useful at certain times and at certain elevations, I generally tend to get as high in elevation as I possibly can, whenever I’m riding. That generally means that no matter where I’m riding, it’s always going to be cold during the night–whether I’m camping at 9,000 feet in the fall or 12,000 feet in mid-summer. I personally underestimated how cold it would get, with overnight lows routinely dropping below freezing. Based on what I now know, I would personally take the 6oz weight penalty and go with the warmest III model.

If you need even warmer temperatures than the Spark SP III offers, Sea to Summit has several other sleeping bag lines that offer warmth down to much lower temperatures.

Photo: Sea to Summit
Photo: Sea to Summit

On two trips I did use Sea to Summit’s Thermolite Reactor sleeping bag liner, in an attempt to combat the extra chill of switching from an enclosed shelter to a tarp shelter. However, it still wasn’t enough added warmth. The Thermolite Reactor itself weighs 8.7oz and adds “up to 14F” to the warmth of the sleeping bag, so I ended up not actually saving myself any weight over the Spark III, and still ended up cold. If you’re on the fence, I’d recommend going with the warmer of the bag options.

Finish Line

With light weight, great packability, and quality construction, the Sea to Summit Spark SP series sleeping bag is the ultralighter’s dream! However, you do need to be realistic with yourself when you choose the temperature of your bag.

Or, you can just do what the experienced ultralighters do: buy one of each temperature for every possible condition.

Thanks to Sea to Summit for providing the Spark SP II Sleeping Bag for review.