I'm with OldBaldGuy on this one. A proper bag, a thermarest, and a closed cell foam pad.

It's amazing how much body heat you can loose through contact with the ground.

Another excellent point was dry a dry sleep kit. Even in temperate weather, I keep one outfit clean and reserved for for sleep (thermals, shorts, whatever). Besides the retarding the "clammy factor" when cold, it really makes a difference in your comfort, and I like to be comfortable when I sleep.