By light, I mean warmest temperature rating.
A bit of background: This summer I'll be headed to Denali with a climbing partner who recently moved to Fairbanks. I exclusively do trad, alpine, and big wall climbing in the lower 48, but I've been to the Alaska Range once before in 2002. My memory of that trip is fuzzy, and my style has evolved considerably since then (one of my first climbing trips).
I err on the side of being light, because I prefer to sleep in all of my insulation (heck, I'm carrying it). In the lower 48, I typically use a 40 degree bag and sleep in my insulation (done winter in the Whites). I'm also resolutely a synthetic guy, so the idea of dragging a -20 degree synthetic up Denali doesn't sound fun.
How light a bag can I get away with if I'm sleeping in my insulation?