Another way to stay warm in addition to using a hot water bottle is to eat right for warmth.
My metabolism and yours are essentially fires that burns within our bodies to generate the energy and power we need to live, learn, think, feel and grow.
Just like building a fire in the fireplace, a wood stove, or in a circle of rocks at a campground, there is a more successful way to build that fire so that it roars to life and burns brightly. You have to put the right materials in at the right times to ensure an enduring fire.
Just as with a fire of flames, the fire in ones belly must be started with something that burns quickly - like newspaper or dried pine needles, leaves and grasses. For the metabolism that would be SUGAR. Sugar catches fast and burns brightly in the metabolic fireplace. So, start with sugar.
The next layer of burnable material after the paper is the small stick layer. For the metabolism this is carbohydrates: grains, bread, pasta - things of that nature.
After the fire is burning well with the paper (sugar), and sticks (carbos) it is time to lay on a few logs so that fire burns slowly and steadily throughout the night. Metabolic logs are fats. Things like cheese, salami, butter, oils.
So to get warm fast using this technique you want to eat a handful of M&Ms, drink some hot chocolate, or maybe a dollop of honey (newspaper). Then eat some carbos - a bagel, some crackers or pasta. Then after you have digested some of that you want to eat a nice hunk of cheese, salami, or some butter.
You would not want to try and warm up by eating cheese first - that would be like putting a big log in the fireplace and trying to light it with a match - probably won't catch and keep you warm. If you start your metabolic fire with sugar, then carbohydrates and then move into the fats you will have a stronger and more guaranteed blaze.
So if it is a cold night and you want to sleep warm try to incorporate this kind of snacking strategy before climbing into your sleeping bag.
SO, the whole system to guarantee a warm and restful sleep would be:
1) heat water for a hot water bottle.
2) do some jumping jacks, pushups, or run in place for a while as the water boils.
3) Pour the hot water into a Nalgene bottle and put it in your sleeping bag.
4) eat some sugar, then carbos, then fats.
5) wear a warm hat
6) jump in your bag.
If for some reason you are still not warm or if you wake up in the middle of the night and are feeling cold, do some sit-ups while inside your sleeping bag and get your blood moving again.
If you incorporate all of these strategies and you have a reasonably warm bag you should be able to stay warm throughout the coldest of nights.