But, Stephen here raises an interesting issue. Yes, it is theoretically possible to wear so much that you compress the sleeping bag's insulation. On the other hand, bags are differentially cut, meaning the inner shell is smaller than the outer shell, making it difficult to fully squeeze the two together. Still, it could be done. I'd also opine that if you're wearing THAT much clothing, you're also constricting your own blood flow, which of course is not good either.
I usually apply the same layering principles in bed that I do when hiking. In cool temps, I'll start with a light layer of long underwear. If that isn't enough, I'll add a vest, light sweater, or expedition-weight long underwear. Hat and gloves and warmer socks, toocold feet, hands, and head account for lots of uncomfortable nights. If I'm still cold, chances are I also have a down parka because I anticipated below-freezing weather. That goes on last, or over the bag itself.
If I'm still cold, well, dawn will come eventually. Or, you can always share a sleeping bag at that point. But honestly, I'd rather suffer a little than share a bag with most of the people I end up in the woods withB
Finally, keep in mind that what is under you makes a HUGE difference. In cold weather, it's often more economical, weight-wise, to carry an extra one-pound sleeping pad rather than a warmer sleeping bag or more clothes.