In moderate weather, why not? An emergency blanket or space" blanket, used in place of a sleeping bag, would keep you pretty warm, assuming youre planning on wearing whatever clothes you already have in your pack. And it makes sense, as you arent lugging around weight that you use only at night. Put on your clothes, wrap up in the reflective blanket, and lights out.
A good example of what to use is the 12-ounce Space All Weather Blanket ($14 at REI). Like all blankets/bags of this type, its designed with a reflective side that directs body heat back into you. Its pretty rugged, so you should get quite a bit of use out of it.
Lighter still, but not as tough, is something like the Space Emergency Bag ($10). This thing weighs only three ounces, but is made from very thin plastic with a reflective coating, so you might not get more than nine or ten uses out of it.
But then, compared to the weight and expense of a sleeping bag, maybe thats not a bad trade-off!