You can also boost the performance of a sleeping bag using a set of down trousers and down gilet, which helps gets around the problem of being to warm for the first couple of hours of sleep then being to cold 4 hours later.

I will also bring along an over sized gilet along with me (Big enough to also get my arms inside). You can sometimes get a lot of down for not too much money. My Karrimor Sub zero gilet cost me around $50 in a sale.

http://www.karrimor.com/karrimor-sub-zero-down-gilet-mens-443974?colcode=44397469

Down Trousers are also available

http://www.jack-wolfskin.co.uk/atmosphere-pants-men/1501141.html

They are very useful if you need to leave the tent to go for a middle of the night time pee.