I second a blanket. A couple of years ago I gave up on sleeping bags because I couldn’t stand the straight-jacket effect. I purchased some SmartWool blankets from Sierra Trading Post. Yes, SmartWool is expensive, but on close-out I paid $35 each (including shipping) for a 51”x71” blanket which weighs 31 ounces on my kitchen scale. I own six altogether, two each for myself, wife, and son.
I first used them when I went camping with my 7-year old son. The night time lows were mild (low 40s), and it was breezy, but not too windy. We slept together in a Sierra Designs Clip Flashlight and used Thermarest ProLites for insulation from the ground (i.e., the blankets were only on top of us, not wrapped around). Since we were together, we could double up the two blankets we had. Turns out we didn’t need to. As the night wore on we kept shedding clothes, finally down to our underwear. After that, we kicked off the blankets we were so hot. In the early morning hours I pulled one blanket back on (my son slept oblivious).
Morals of my story: (1) I and my son both sleep hot, so a little insulation goes a long way; (2) a good tent traps heat; (3) if you’re with someone, the shared heat will make a big difference; (4) separate ground protection helps a lot (a sleeping bag compressed under the weight of a person doesn’t offer much insulation on the bottom); (5) the weather cooperated. In short, there’s a lot of individual variables that will probably affect your results.
I love the SmartWool blankets. I’m not straight-jacketed. They breathe. If they get wet, they still work. They don’t melt like synthetics. I especially like that they’re not limited to camping use—I try to buy gear that’s not too specialized to get maximum utils. If I expect really cold weather, I take two; otherwise I take one. In any case, I take an AMK bivvy as insurance in the day pack which rides at the top of my Kelty 50th Anniversary external frame pack. The bivvy’s job is the same as the tent’s: trap the heat. Yes, the bivvy would trap me like a sleeping bag, but then the bivvy is for emergency use, not casual use.