When the influenza virus takes hold, the body mounts a defense that often includes a high fever. This immune response can lead to a confusing cycle of feeling chilled one moment and waking up drenched in sweat the next. Understanding the physiological mechanism and its meaning for the illness can help demystify this common experience.
How the Body Regulates Temperature During Viral Infection
The body’s temperature control center, a small region in the brain called the hypothalamus, acts as a thermostat to maintain a stable internal temperature. When the body detects a viral intruder, immune cells release signaling molecules called pyrogens. These pyrogens travel to the hypothalamus and trick the “thermostat” into raising its temperature set point to a higher level.
The sudden increase in this set point causes the body to perceive its current temperature as too low. To reach the new, higher set point, the body initiates heat-generating and heat-conserving responses. These mechanisms include shivering, which uses muscle contractions to produce heat, and peripheral vasoconstriction, which narrows blood vessels near the skin to reduce heat loss. These actions are responsible for the chills experienced during the onset of a fever.
Once the body temperature reaches the new, higher set point, the fever stabilizes, and the chills typically subside. The sweating phase begins when the hypothalamus resets the temperature set point back toward the normal range. This downward reset can happen naturally as the immune system gains control over the virus or artificially after taking a fever-reducing medication.
Since the body’s actual core temperature is still high, the hypothalamus activates cooling mechanisms to rapidly drop the temperature to the new, lower set point. The primary cooling method is the dilation of blood vessels near the skin’s surface, allowing heat to radiate away, combined with the activation of sweat glands. As sweat evaporates from the skin, it transfers heat away from the body, leading to the drenching perspiration that accompanies the end of a fever cycle.
What the Sweating Phase Means for Illness Progression
The onset of heavy sweating is a positive indication in the progression of a flu infection. This physical sign means the body has entered the phase known as defervescence, or the “breaking” of the fever. The immune system has successfully advanced in its fight, prompting the hypothalamus to lower the set point back toward a healthy baseline.
This sudden cooling response marks a turning point where the body is actively reducing its core temperature, signifying that the period of high fever is ending. The sweating is a byproduct of the body’s successful attempt to regulate itself and bring the internal temperature back to a normal, non-fever range. For many people, this transition occurs around day three or four of the illness, when the immune system has largely gained control of the viral replication.
When a fever breaks, it often signals a slight easing of overall body aches and discomfort, though other flu symptoms like congestion and fatigue may persist. The intensity of the sweating corresponds to how high the fever was, as a larger amount of excess heat needs to be dissipated. While the sweating is an encouraging sign of recovery, continued support is necessary for healing.
Preventing Dehydration and Recognizing Emergency Signs
The profuse sweating that occurs when a fever breaks can lead to significant fluid loss, making dehydration a serious concern during the flu. The increased metabolic rate from the fever itself also contributes to fluid loss. Maintaining adequate hydration is necessary to support recovery and prevent complications.
Consume plenty of fluids, such as water, broth, or electrolyte-rich drinks, throughout the illness, not just during the sweating phase. Signs of mild to moderate dehydration include increased thirst, a dry or sticky mouth, and urine that is darker yellow than usual. Frequent, small sips of fluid are often better tolerated than trying to drink a large amount all at once.
While sweating is usually a sign of a resolving fever, it is important to watch for signs that indicate a medical emergency. Seek immediate medical attention if you experience severe dehydration symptoms like confusion, dizziness, or an inability to keep fluids down. Other emergency signs include:
- Difficulty breathing.
- Chest pain.
- A seizure.
- A fever that persists beyond three days or returns after initially disappearing.
These symptoms suggest a potential complication, such as pneumonia, that requires immediate professional care.