I sleep pretty much like Ken, tossing and turning a fair bit, but favoring my stomach or side. My main bag is a hoodless bag (Feathered Friends Vireo) and it does quite well for me when combined with a jacket (which I would carry anyways) in the spring or fall or at higher elevations. It's also fairly important to have a good hat along. Depending on the weather, I may have a light merino cap, an Integral Designs Primalid, or even the Jacks 'R' Better down hood. Again, I'd already be carrying the hat anyways.
Roger's point about the importance of having insulation for the head is very important, however I wouldn't go so far as to say that hoodless bags are only good about 80 degrees. I like my Vireo and think it's great for many kinds of trips.