I attributed comfort beneath opened summer weight sleeping bags to extra loft from extra width, wool sweaters, and conservative ratings from the makers. Or to being a "warm sleeper."
After a couple of comfortable nights, without the wool, beneath a Hammockgear 40F -rated top quilt, to 22F, I'm not so sure.
I wonder whether the super-adequacy of underquilts is not the source. The notion / hypothesis is that UQ - hammock combinations are unique in sleeping experience. There's no heat sink and heat loss is lower than what most people experience even in their own beds by conduction unless they use lots of insulative padding between themselves and their mattresses.
In my case, the UQ has been a short full-length Hammockgear underquilt, above which I haven't been cold to 10F either.
What seems to make a down UQ work so well is its low mass, not just its resistance to heat flow. If the slightest gap has formed between the UQ and the hammock bottom on a cold night, and I pull the UQ up snug, I can immediately feel a bit of cold from the transfer of heat from my skin to the smallest of heat sinks, the top layer of the UQ. And then, I am immediately warmer.
What is your experience with TQ ratings, sleeping above a properly working underquilt? Do you sleep comfortably under thinner TQ than you expected for the temps?