Sewed up my first hammock (double layer, gathered end) earlier this week, and have been testing it in the garage, sleeping in it happily at 60-65 degrees. This weekend was the first night out in the great outdoors.
My son and I hiked to the top of a peak near Tucson. 7200 feet and plenty of trees.
Temperatures at night dropped to about 45 degrees, almost no wind.
On top I had my trusty JRB High Sierra Sniveler quilt, which has been part of my ultralight backpacking setup for a couple of years. Between layers went a 39x59 by 1/4 inch "thinlight" pad I bought from Gossamer Gear for hammock use. Quickly it was clear that that pad was not sufficient for the conditions (though I think the quilt is fine). I would have shivered all night - so I got up and pulled the pad out, folded it in half so it was 29x59 and now 1/2 inch thick. This was definitely better, but cold along the edges. I could sleep, but was not snug and happy (and had to take care to lay in just the right place).
I tend to sleep cold. I don't want to carry a bunch of gear backpacking (I have bought into the ultralight thing heavily, and love it). So I am pondering what to carry on the next trip. As soon as it is clear that there is no turning back, I expect to invest in an under quilt, but for now what I am thinking is that I will need to take two pads. The already mentioned 1/4 inch pad along with a torso length thermarest I have. I will spread out the 1/4 in pad fully to get insulation on the edges and put the thermarest dead center under me where most of my weight presses down, so both pads will insulate me there (I have tested this in the garage and it is not lumpy and weird). A real cold weather test will come later this week or next weekend.
But you veterans may have wise advice! ....