What he says. UQ's tend to be lightweight and lack much if any windproofing. Hence they're more likely to lose heat when cold air circulates in the relatively exposed / drafty set-up of a hammock tarp. That's after you've ensured there's as little "flushing" of warm air by having a poorly fitted or not-cinched-up UQ. Had a very unpleasant night on my first trip with the WB Ridgerunner and a frigged UQ set up. It was like being flushed by a wave when surfing - a cold wash as all the warmth disappeared - you can't fall back on pissing yourself like in a wetsuitTaylor wrote:@browndog
An under quilt protector makes a big difference for me.

You could try a hammock sock as well which all helps to trap air and reduce movement. You can also get over-covers to fit some hammocks. They don't replace a tarp usually but trap warm air in and reduce drafts.
If you've a shorter style UQ think about doubling up with insulation (e.g. a mat inside the hammock - only partially inflated to let it shape round you easier - personally I find mats a PITA but others manage) or using some CCF pads where the UQ doesn't cover e.g. feet and/or head. I've occasionally shoved a small bit in the closed footbox of my TQ to keep it in place. You could even use a "full-size" T-shape (for shoulder coverage) CCF as folks used to.
Do you use a hat? TQs don't usually have hoods like a normal sleeping bag. A cold head is pretty miserable, IME.
There's a bloke called Shug (IIRC) in the States who's done loads of Youtube stuff on hammocking. I remember watching one he did about cold hanging - worth a look.
"Wastes" fuel but there's also the "hot water bottle" option. Placed in the groin (apparently warming the blood in the big vessels there - carotid) is supposed to be effective.
And after all that I also think that when the temps drop staying warm whilst hanging can simply be that bit more of a challenge. Sometimes going to ground is the *right* thing to do.