Viral fever usually increases at night, or the symptoms get worse. The cortisol levels in the blood decrease at night, which is readily sensed by the immune system and white blood cells start to fight the viruses in your body. As a result, more pyrogens are released into your body, elevating the body temperature, and the fever goes up. This could also be associated with typical fever symptoms such as chills and hot flashes. For more information on viral fever, reach out to our medical experts.