Hot Rock Bed or Firebed
(Rocks or coals on the left and covered with earth on the right)
Hot Rock Bed
To help you to keep warm while lying down you can create a trench and fill it with hot rocks that were heated in a fire. You then cover the trench with earth.
Be certain that the rocks weren't taken from a stream or other body of water, because if the rocks are water-logged, the heat will cause the water in the rocks to expand and the rocks can explode. This is a very real situation that can happen, and it could take out an eye with flying fragments of sharp rock.
An alternative to the hot rock bed is to make a fire in the trench, then when it is down to coals to cover it with earth.
This will raise the temperature of the ground you will be sleeping on. Put a bedding on top of the area where the trench or bed is.
The image above is just for illustration purposes. Don't use a 1000 rocks in a way of putting it if making a hot rock bed. A firebed is preferable, easier and potentially safer given that some rocks can explode when heating them if you don't choose the right ones. And you have to fetch several rocks and heat them up which is more time consuming than fetching wood and making a fire for the firebed. The firebed will remain warm longer as well. Make use of the fire on the firebed for boiling water and keeping warm while attending to other tasks at camp, then cover it up with earth or sand depending on where you are.
If you do not make a hot rock or firebed, you may use heated up stones to help you to keep warm at night by placing them around you as the heat will radiate from them. Keep a regular batch of rocks in the fire to rotate those you are using as they cool off to always have hot rocks to use.
You can get some rocks from overturned trees that have their roots exposed. It goes better to have an axe or other to get at them if they are stuck in ice.
Here are some links to help you learn more about hot rock beds and firebeds.
1. Trails.com - How to Make a Fire Bed
2. How To Make A Firebed - No More Dependence
Have a look at the Videos page as well for more information on the subject.