Body temperature is really hard to tell in an accurate way, even if we have some temperature sensor very close to your body: the sensor would be influenced a lot by the type of mattress.

The interesting one for circadian cycles is the "Core Temperature" but it's rather uncomfortable to measure (see http://en.wikipedia.org/wiki/Human_body_temperature#Core_tem...), what we can do instead is measuring your heart rate at rest that is something typically constant for a healthy person: if that varies from the usual value more than about 5bpm it usually means you have been exercising A LOT or that you are sick.