The part of your brain that regulates this is called the hypothalamus. It knows what temperature your body should be (usually around 98.6 °F/37 °C) and will send messages to your body to keep it that way. Most people's body temperatures change a little bit during the course of the day. It's usually a little lower in the morning and a little higher in the evening. In kids it can vary as they run around, play, and exercise. Sometimes, though, the hypothalamus will ““reset”” the body to a higher temperature in response to an infection, illness, or some other cause.