Sleeping bags are designed to retain the heat generated by your body while you sleep to keep you warm. If you're camping in a chilly environment and the bag by itself isn't quite cutting it, you can generate some heat the old-fashioned way -- with a little exercise before you sleep.
Photo by Sayan Nath via Unsplash.
Do what you can to warm your body before you get in the bag. Twenty jumping jacks before you hop in the bag may keep you warm hours longer, because your body will better warm the air in the bag.
If you exercise and them immediately get into your sleeping bag, it can warm the air in the bag and keep you a little more toasty hours after the excess heat from your workout has already dissipated. That means you should obviously zip up the bag as much as you can to retain the warm air. Ideally you also have a sleeping pad so that your heat doesn't escape into the ground. It may be common knowledge to experienced campers, but I'll definitely try a little workout before bed next time I'm roughing it.