A fever is an elevated body temperature that goes above the normal range of about 98.6°F (37°C). The experience often involves a confusing cycle of feeling both hot and cold. This dual sensation occurs because the body’s sophisticated internal temperature regulation system is actively fighting an infection. It is a normal physiological process as the body attempts to reach a new, higher temperature set point and then return to normal.
The Body’s Thermostat
The body’s temperature is managed by the hypothalamus, a region in the brain that acts like a thermostat. This structure receives input from temperature sensors and maintains a narrow temperature range for optimal function. When an illness begins, the immune system releases signaling molecules called pyrogens. These pyrogens travel to the hypothalamus, effectively “tricking” the thermostat into raising the body’s temperature set point. For example, if the normal set point is 98.6°F, pyrogens might reset it to 102°F. This intentional temperature increase is part of the immune response, as the higher heat can inhibit pathogen growth and enhance immune cell activity.
Why Shivering Makes You Feel Cold
Once the hypothalamus resets the temperature target to a higher level, the body perceives its current temperature as too low. Even if the actual temperature is a normal 98.6°F, the brain signals coldness because it is aiming for the higher set point. The body initiates responses designed to generate and conserve heat to meet this new target. One immediate response is peripheral vasoconstriction, where blood vessels near the skin’s surface narrow. This action shunts warm blood away from the extremities and toward the body’s core, which helps conserve heat internally.
This process causes the skin to feel cool or clammy to the touch, and it contributes to the sensation of feeling cold. The body also begins shivering, involving rapid, involuntary contractions of skeletal muscles. This muscle activity efficiently generates heat to rapidly raise the core temperature. The combination of vasoconstriction and shivering causes intense chills, which persist until the body temperature successfully reaches the new hypothalamic target.
Why Sweating Makes You Feel Hot
The sensation of feeling hot and the onset of sweating signals that the fever is beginning to break. This phase occurs when the source of the infection is cleared, or when fever-reducing medication is taken, causing pyrogens to stop acting on the hypothalamus. The hypothalamus resets the temperature target back down to the normal range, typically around 98.6°F. At this point, the body’s core temperature is still elevated, but the thermostat is set much lower.
The body perceives its current temperature as too hot and must actively dissipate the excess heat. The primary mechanism for cooling is vasodilation, where the blood vessels near the skin expand again. This widening of the vessels brings large volumes of warm blood to the skin’s surface, causing a flushed, hot, and sometimes visibly red appearance. Heat is then released into the environment, a process significantly enhanced by sweating. Evaporation of the sweat rapidly lowers the core temperature, making the person feel intensely hot.
Comfort Measures and Warning Signs
Comfort Measures
Managing the sensations of a fever involves adjusting comfort measures. During the cold phase, when chills and shivering are present, light blankets and a warm drink offer comfort. Avoid bundling up excessively, as this can impede temperature regulation and cause the fever to spike higher. When the hot, sweating phase begins, the focus shifts to heat dissipation. Removing excess clothing, using a light sheet, and applying a cool compress can help relieve the intense heat sensation. Staying well-hydrated by drinking clear fluids is necessary, as profuse sweating can quickly lead to dehydration.
Warning Signs
Although most fevers resolve naturally, certain symptoms warrant immediate medical attention. Seek care if a fever is accompanied by severe symptoms, such as a stiff neck, confusion, or extreme irritability. Other warning signs include difficulty breathing, an intense headache, or the inability to keep any fluids down due to repeated vomiting.