Your body's constantly radiating heat to the environment (unless of course you're in an environment of 98.6°F or hotter). Since your body is maintained at a constant temperature (ignoring possible illnesses which can raise or lower temperatures) the thing that does this is the energy you intake. A single food calorie is enough to raise your one kg of water by one Celsius degree (or 1.8 Fahrenheit degrees). Since your body's 60% water, if you weigh 60kg then it will take you 36 food calories to keep your body from cooling to 96.8°F.
Now obviously I can't say how fast your body radiates heat (I'm sure an experiment could be performed to figure it out, and that would be kind of interesting), but you could do something simple like take one liter of hot tap water (should be around 120°F depending on your water heater) and take the temperature every 30 seconds to find out how long it takes to drop to ~99, then below. That could give you an idea of just how much energy is used to keep you warm.