Mine was doing the same thing- while experiencing very bad battery life. Full restore fixed it, but you might want to try some less drastic measures first, Europa laid those out in the thread "battery life worse than ever" on the iPhone 5 forum. Good luck
Hold power and home button until you see the Apple, that should clear the problem out, if that doesn't work, you may have to do a full restore. Restore as new and not from backup if you have to go this route.