Sea to Summit Spark SP III Sleeping Bag Review: light and warm and packable

The Sea to Summit Spark SP III Sleeping Bag is part of the Sea to Summit Spark series of unisex down sleeping bags (the company also has a complementary women’s specific line called the Flame). Both ranges are designed to be ultra-light and technical, offering impressive warmth ratings at relatively low weights.

The Sea to Summit Spark SP III Sleeping Bag is available now, priced at £450 for the regular size, and although that's a serious price, it is one of the best lightweight sleeping bags available today – in fact, it's among the best sleeping bags on the market, full stop.

Sea to Summit Spark SP III (3) Sleeping Bag Review: Design and Build 

Being a technical bag, the Sea to Summit designers have taken the obvious choice and gone for a down filling, in theory offering maximum warmth for minimum weight. The down is Ultra-dry treated Responsible Down Standard (RDS) Premium Goose Down, at a substantial 850+ Loft, and a fill weight of Regular: 430g, Long: 485g - which in the case of the regular is about two-thirds of the total bag weight.  

 Speaking of bag weight, we have pleasant news in store - the Regular size on test here comes in at a massive 665g, or roughly the weight of a full pint of beer. That’s really rather light, putting the Sea to Summit Spark SP III Sleeping Bag into the top percentage of lightweight/ultralight sleeping bags around. There’s more good news too, as the Sea to Summit Spark SP III Sleeping Bag sits around the middle of the range (the Spark zero weighs in at 225 grams), which makes it heavier, but beefier in warmth terms - we’re talking  -2°C Comfort to -8°C Lower, which for the weight is almost laughable. 

Many a lightweight bag will achieve a low weight by shortening the zip - a fact that does cut weight, but adds considerable annoyance when actually sleeping in it - but the Sea to Summit Spark SP III Sleeping Bag does no such thing, featuring a perfectly normal three-quarter length zip. 

 Indeed, the only visible hint that this bag is stupidly light comes from the fabric, which is indeed somewhat thinner than you might find on more robust bags - we’re talking 10D Nylon shell, 7D liner, which isn’t gossamer, but does feel lightweight to the touch. It’s also almost see-through, which adds to the ultralight aesthetic. That said, there’s nothing immediately worrying about the bag’s build quality when handled - it’s just very light, which is very much the point.

It’s also very much a premium bag, coming with a lightweight compression sack and a larger storage bag, as well as a mini-biner to attach the provenance documents.  The design is a relatively standard box-baffle construction, with vertical sections over the torso, and there’s a draft tube over the zipper to beat cold spots. Strangely there’s no neck baffle, which is normally a go-to when boosting the warmth of a bag.

Sea to Summit Spark SP III (3)Sleeping Bag Review: Performance and comfort 

The comfort on offer with the Sea to Summit Spark SP III (3)Sleeping Bag is considerable, due to the simple combination of three things - tactile materials, proper length zip and that high-fill-power down loft. The latter is really very premium, giving an immediate warmth once you climb in - the insulation hugely helped by the premium box wall construction, which helps prevent coldspots as well as allowing lots of loft.  

 Space within the bag is relatively generous, leading to a tardis-like impression - Sea to Summit Spark claim they’ve sized the Sea to Summit Spark SP III (3) up to cope with expedition-style layering, and that was certainly the case for our testing. The downside of this roominess is the draft around the neck, which without a baffle is hard to entirely cut out. 

Similarly, the zip tube is on the smaller side, which combined with unfortunate down migration, does leave the zip as a bit of a cold spot at times. An upside of this thinking is that the footbox is roomy, allowing tired feet the chance to decompress in comfort - a pleasant change from some bags that take the ‘mummy’ cut to extremes. Another plus is the zip guard, which operates faultlessly, allowing casual, one-handed zipping and unzipping at your pleasure - a trick that many otherwise excellent bags fail to replicate.  

In outright performance terms the temperature gradings might be a touch on the optimistic side, but then to be fair that’s a highly subjective question, and historically the concepts of ‘warm’ and ‘ultralight’ were very much strangers. The Sea to Summit Spark SP III (3)Sleeping Bag is very much a warm and comfortable bag, but in colder moments you can start to feel the edges - which with a -2°c Comfort rating is understandable.  

Sea to Summit Spark SP III (3) Sleeping Bag Review: Verdict 

Let’s get this straight out - the Sea to Summit Spark SP III (3)Sleeping Bag is a really lovely bag. It feels exciting and technical to the touch, and is bafflingly light without losing any of the space or key design elements you’d want from a sleeping bag. It lofts beautifully and luxuriously in double-quick time, and we had no issues sleeping happily in low-single-figure temps. The proper zip means that venting is easy, and essentially the whole experience is the same as a much heavier ‘normal’ sleeping bag, just with half the weight to tote around.

That weight saving is really the heart and soul here - sure, there are warmer bags, and they weigh at least double this (and some plenty more). However, if you’re carrying this long distances, or engaged in long, off-season days out that might culminate in a hut or bothy, that lack of weight will power you all day long, and you’ll still get a comfy bed at the end of it. 

Overall, the Sea to Summit Spark SP III (3) Sleeping Bag is not a cheap sleeping bag, and is complete overkill for car campers and occasional outdoors venturers, but if you’re in the market for a technical, easily-carried but warm sleeping bag, this is absolutely one to take a look at - just don’t let the crampons get near it.