Is there a light wearable sleeping bag out there?

Not exactly your question, but Jacks R Better has quilts that are wearable and usable in that range. I used their Sierra Sniveller in Scotland last month in those temperature ranges (their web page rates it as 25°-30°). One night it got down to 7 dec F when we were higher up in the hills and I was ok with most of my layers on, and I’m generally a cold sleeper.

Exped, Feathered Friends and a few others.

I used the Exped WallCreeper for a few years, but found that a down hooded sweater/jacket combined with a Summer sleeping bag was just as warm, but more flexible.

20 degree and about 30 oz Nunatak . Here is a video of a custom zero degree Raku.

I use a Feathered Friends Rock Wren with the optional 2oz of overfill. I have had it down to 30 degrees and been comfortable in my merino sleeping cloths, but I sleep warm. I have also worn it around camp in light rain and wet snow. Mine is older version with an EPIC shell. As a side sleeper, and a big guy, I love being able to have my arms out and love the extra volume. At 29oz, it is a bit on the heavy side, but it saves me a bivy, as I use a tarp, and it saves me carrying a down jacket for camp. Feathered Friends makes the Winer Wren that is good down to 25 degrees and more if you added overfill. Their quality is excellent.

Generally you need more insulation when sleeping than when up and maybe about at camp.
(also at around 4-5 AM it is usually colder than at 7-9PM)
For that I think a layering system , say puffy jacket and pants and a lighter sleeping bag, works best.
For example for about the same weight as a WM Ultralite alone I use the WM Flash pants and jacked with the Summerlite , the pants and jacket giving me more mobility and layering options with other day clothes than having a wearable bag alone.
Not that I have tried a wearable bag…

I love wearing my enLIGHTened Equipment quilt as a cape….does that count??

Seriously, I have owned and LOVED the Exped Wallcreeper/Dreamwalker bags. Just awesome!

http://www.exped.com/exped/web/exped_homepage_na.nsf

A picture of me making breakfast on a cold morning wearing a Wallcreeper:

Great advice. Checked them all out. A little too heavy for now. May get zpacks quilt at 20 degrees to add to lightness of load (rather than heavier 15 degree bag I carry).

The Jacks r Better quilt/ serape combo immediately cam to mind. You can wrap just about any quilt around you in camp while sitting or standing around.

Another option is to use a puffy jacket with a short sleeping bag called a half bag or elephant's foot bag. But you'll note they aren't mentioned much here. The combo can be quite expensive, and you often end up with a jacket that is overkill for most 3-season use. As another posted, you tend to need different levels of insulation for sleep and waking use.

If you are concerned with weight, quilts or high end down bags are the lighter option. You can go with an even lighter bag if you coordinate your clothing to use as sleep insulation as well. No free lunch on any of the options.

If you do a lot of multi-day trips, you may find that having a couple different bags is the best easy to cover the temperature ranges.