I like Ron's information.
My cold-weather backcountry bed consists of 3 components: A LuxuryLite UL cot keeps me about 4" above hard ground and is incomparable for comfort. On the cot goes my Exped Downmat 9 which provides about 3.5" of down insulation beneath my body, plus the comfort of an air mattress. My bag is a Western Mountaineering Sequoia GWS which comes standard with 36 ounces of 850 fill-wt down in the 6'-6" model. It's rated to 5F, standard. I ordered mine custom with 4 extra ounces of down, which likely takes the bag rating to well below zero. When placed on top of the Downmat and its additional warmth, I've got the closest thing to a true furnace. I'm often venting on cold nights in central Alaska.
A thin warm beanie will add an estimated 5-10 degrees of comfort to your bag...especially if you're not sporting a thick mop up top.
I always sleep in long underwear, socks and a beanie. I do it mostly to protect my down bag from skin oil and other contaminants which will reduce a bag's loft over time. I also much prefer jumping up at 5am when I have the first basic layer in place. Don't do this in cheap long underwear. Use the best synthetics or merinos.
I absolutely concur with using a pee bottle to eliminate the jump-and-run at 3am. Without going graphic, I've figured out how to safely (and cleanly) take care of that job while remaining comfortably in my sleeping bag. No more night trotting.
Do sleeping bag maintenance in camp. Open it up every day before leaving to hunt. On sunny days, hang it outside (inverted) on a paracord clothesline to totally dry the insulation or fill.
You spend almost 1/3 of any hunting or camping trip relying on your bed system. It makes no sense (to me) to go cheap or uncomfortable to save a few bucks. Buy good stuff and give it the care it requires.