Shivering is an involuntary, rapid muscular response that the body uses to generate heat. This automatic process is fundamental to maintaining a stable internal temperature, a state known as thermoregulation. The muscular contractions are unsynchronized, meaning they do not produce coordinated movement. Instead, they convert stored energy directly into thermal energy, increasing the body’s internal heat production.
The Body’s Heat Production Mechanism
The central nervous system orchestrates the shivering response, with the hypothalamus, a small region in the brain, functioning as the body’s primary thermostat. This area constantly monitors the core body temperature and receives signals from temperature receptors located throughout the skin and spinal cord. When these signals indicate a drop below the acceptable set point, the hypothalamus initiates a cascade of warming responses.
The posterior region of the hypothalamus contains the primary motor center for shivering, which sends descending signals to the skeletal muscles. This process is highly inefficient, intentionally wasting the chemical energy stored as Adenosine Triphosphate (ATP) to produce heat as a byproduct of movement. Maximum shivering can dramatically increase the body’s basal metabolic heat production by three to five times in a short period. As the core temperature rises and the blood reaching the hypothalamus warms, the command signals cease, and the involuntary muscle contractions eventually stop.
Differentiating Triggers: Cold vs. Fever
Shivering can be triggered by two fundamentally different scenarios: an external cold environment or an internal infectious process. When exposed to cold, the peripheral temperature receptors signal the hypothalamus that external conditions are causing heat loss. The resulting shivering is a homeostatic attempt to raise the current, normal core temperature back to the standard 98.6°F (37°C) set point.
The mechanism during a fever, however, is distinct because the body’s thermostat is deliberately reset to a higher temperature. Immune cells release chemical messengers called pyrogens, such as Prostaglandin E2, which act directly on the hypothalamus. These pyrogens elevate the set point, sometimes several degrees higher, convincing the body that its current temperature is too low relative to this new target. Because the core temperature is now below the new elevated set point, the body initiates the heat-generating processes of vasoconstriction and shivering. Once the core temperature reaches this elevated set point, the shivering stops, and the fever is established.
When Shivering Indicates a Serious Problem
While often a normal thermal defense, shivering can also be a symptom of a serious medical condition that requires immediate attention.
In cases of hypothermia, defined as a core temperature below 95°F (35°C), shivering occurs in the mild stage as the body attempts to compensate for heat loss. Paradoxically, the cessation of shivering in moderate hypothermia is a dangerous sign, indicating that the body has depleted its energy reserves and can no longer generate heat.
Shivering, or rigors, can also be a sign of systemic infection, such as sepsis, which is the body’s life-threatening response to an infection. The massive inflammatory response associated with sepsis can trigger an uncontrolled febrile response, leading to severe and prolonged chills.
Furthermore, a sudden drop in blood sugar, or severe hypoglycemia, can also induce shivering because the central nervous system lacks the necessary glucose for fuel. The symptoms of low blood sugar, which can include trembling, sweating, and confusion, mimic the body’s response to cold exposure. This response is a systemic attempt to stimulate the release of counter-regulatory hormones to raise blood glucose levels, serving as a warning sign of metabolic distress.