Sea to Summit Spark III is the benchmark for ultralight sleeping bags. We researched its 34°F rating accuracy and compared its packability to competitors.
Who it's for
Buy this if: Fastpackers and thru-hikers who need a sleeping bag that disappears in their pack.
Skip this if: Cold sleepers — the 34°F comfort rating requires a healthy warm sleeping temperature; cold sleepers should go for Spark II (28°F) or add a liner.
In practice
Thermal Rating Reality
Sea to Summit uses ISO 23537 standard ratings. The 34°F comfort rating applies to a 'European standard woman' at rest — inherently a conservative rating. In practice, warm sleepers in a baselayer found the Spark III comfortable at 30°F. Cold sleepers: add the Sea to Summit Silk Liner for an extra 8°F.
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