I vote for a warmer sleeping bag; and I would go for a -10F to -20F rated bag for that time of year. The reason is you can always unzip the bag and hang a leg out if you are too warm but being cold is miserable as well as potentially life threatening. The way sleeping bags are rated can be deceiving as some are rated for survival and others comfort. Other factors are sex, health, body structure, when you last ate (body fuel), and environment. Here is a simple guide;
You can find more online with a simple Google search.