Proper insulation from the ground is more important than the stuff on top!
You lose heat 25% from the top and 75% from the ground so by rights your insulation from the ground should be three times as much as your cover. You have to take into consideration compression too... an if you toss and turn at night, you're digging yourself down further. Everybody makes that mistake!! You can spend a night out in sub-zero temperature if you're properly insulated from the cold ground. A few pointers on natural bedding insulation....
-box up your bed with logs if you can to minimize spreading out your debris.
-don't worry about finding the most supple pinebows... if its thick enough even large diameter sticks will be comfortable... plus they add airspace.
-don't use snowy/damp/frozen leaf litter... of you'll wake up frozen to the ground... lol.
-tying or bundling your insulation really helps when using grasses and reeds.. also laying them in a criss-cross patern keeps you off the ground.
-if you are in a situation where you can't get enough insulation under you to be comfortable, lay on your stomach to protect your kidneys and spine from the cold... you will be better off in the morning.
One of my favorite tricks is to fill a poncho with grasses, reeds, leaf litter... fill it until its ready to pop! .... its like heaven on a cold night plus the ponch keeps any moisture from the litter off of you...
This was AFTER spending a cold night on them.... they were round when we turned in.
Rick