When you feel hot or cold, you are actually feeling the temperature of your blood. Since you are considered a warm-blooded animal (along with birds and other mammals), your body maintains an average temperature of 98.6° Fahrenheit (37.0° Celsius), no matter what the temperature is around you. Each species of warm-blooded animals has its own normal body temperature. Your normal body temperature is maintained by a part of your brain called the hypothalamus.
The hypothalamus, along with controlling hunger and thirst, is actually a “temperature center,” which receives messages from nerves in your skin and deep in your body. It regulates the temperature of your blood by burning food to generate body heat when the air outside is cold, and cools your blood when the temperature outside is too hot.
Shivering and other physical activity also generate body heat in cold weather, while sweating and panting cools you off in hot weather. So whenever there is a change in the temperature of your blood, your body makes some adjustment to get it back to normal.
Cold-blooded animals (all those except mammals and birds), on the other hand, do not have this built-in brain control over their body temperature and must resort to other means. Reptiles, for example, crawl into the sun to warm up and seek the shade when they are hot. Some fish increase the action of their muscles to generate body heat.
Your body temperature does vary during the day. It is lowest in the morning, rises in the late afternoon, and falls again while you sleep!