im sure that the Bluetooth is inactive when the screens locked, unless you have a device connected. this saves battery. I have tested this theory by leaving Wifi Bluetooth 3G on all day and the battery hasn't been adversely affected.
also a great way to get extra battery is to turn off the mobile data and wifi, phone cant update and so the battery just keeps on going. coupled with light usage I managed 3 days (only turning off at night ).
(nokia lumia 800)