Sleeping bags keep you warm during the night by trapping and keeping hold of a layer of 'dead air' which...
Sleeping bags keep you warm during the night by trapping and keeping hold of a layer of 'dead air' which is then warmed by your body heat. This warm air then lies against the body to keep you warm and comfortable and forms a barrier between the ground and colder air outside the bag. The less air space there is around your body the faster you will warm up and stay that way, which is why mummy bags are the perfect bag for camping.
Mummy bags are tapered against the body so there is less dead air space and this reduces cold spots forming. Standard family camping bags will be a little looser in fit than a backpacking bag so look out for this. If you prefer a little more room in your bag and move a lot at night and you aren't out in cold temperatures or technical camping then a mummy sleeping bag might not be for you.
Mummy bags can be tailored to your needs so look out for women specific or childrens mummy sleeping bags. Many brands recognise that women tend to be shorter in height than men and so don't need as much length to the sleeping bag. Any extra length will just allow for cold spots to form and means you'll be colder in the night. Some brand such as Vango provide childrens bags that are convertible which means the length can be alterered. These are great when kids grow from year to year as the bag can just be extended each year and will last longer.
Many mummy sleeping bags that are more technical will include a hood. These are brilliant for cinching around the head and shoulders to keep that warmed air and body heat in.