Shivering (also called shuddering) is a bodily function in response to cold and extreme fear in warm-blooded animals. When the core body temperature drops, the shivering reflex is triggered to maintain homeostasis. Skeletal muscles begin to shake in small movements, creating warmth by expending energy. Shivering can also be a response to fever, as a person may feel cold. During fever, the hypothalamic set point for temperature is raised. The increased set point causes the body temperature to rise (pyrexia), but also makes the patient feel cold until the new set point is reached. Severe chills with violent shivering are called rigors. Rigors occur because the patient's body is shivering in a physiological attempt to increase body temperature to the new set point.

When understanding the concept of shivering it is important to understand the human physiological responses to cold exposure. There are many reactions that humans have to the cold such as “cutaneous vasoconstriction and shivering thermogenesis” these processes “decrease heat loss and increase metabolic heat production”. ^[1] 

Cutaneous vasoconstriction is the narrowing of blood vessels in the skin reducing blood flow and heat loss to environmental temperatures. This process is one of the many natural defenses' humans have against cold temperatures.  

Thermogenesis (shivering) is when the body comes in contact with cold temperatures in the environment and the body feels as though it needs to raise its core body to protect itself. This is a primary trigger for the mechanism of heat production in mammalian bodies when exposed to cold. This process generates heat though high speed contractions of the skeletal muscles. It begins by having the hypothalamus triggering a signal in the muscles to contract and relax making heat the excess product of the rapid muscle movement that the process causes.^[1]  

Located in the posterior hypothalamus near the wall of the third ventricle is an area called the primary motor center for shivering.^{[citation needed]} This area is normally inhibited by signals from the heat center in the anterior hypothalamic-preoptic area but is excited by cold signals from the skin and spinal cord. Therefore, this center becomes activated when the body temperature falls even a fraction of a degree below a critical temperature level.^{[citation needed]}

Increased muscular activity results in the generation of heat as a byproduct. Most often, when the purpose of the muscle activity is to produce motion, the heat is wasted energy. In shivering, the heat is the main intended product and is utilized for warmth.^{[citation needed]}

Newborn babies, infants, and young children experience a greater (net) heat loss than adults because of greater surface-area-to-volume ratio. As they cannot shiver to maintain body heat,^{[citation needed]} they rely on non-shivering thermogenesis. Children have an increased amount of brown adipose tissue (increased vascular supply, and high mitochondrial density), and, when cold-stressed, will have greater oxygen consumption and will release norepinephrine.^{[citation needed]} Norepinephrine will react with lipases in brown fat to break down fat into triglycerides.^{[citation needed]} Triglycerides are then metabolized to glycerol and non-esterified fatty acids.^{[citation needed]} These are then further degraded in the needed heat-generating process to form CO₂ and water.^{[citation needed]} Chemically, in mitochondria, the proton gradient producing the proton electromotive force that is ordinarily used to synthesize ATP is instead bypassed to produce heat directly.^{[citation needed]}

Shivering can also appear after surgery. This is known as postanesthetic shivering.

In humans, shivering can also be caused by cognition.^[2] This is known as psychogenic shivering.^[3][4]

The functional capacity of the thermoregulatory system alters with aging, reducing the resistance of elderly people to extreme external temperatures. The shiver response may be greatly diminished or even absent in the elderly, resulting in a significant drop in mean deep body temperature upon exposure to cold. Standard tests of thermoregulatory function show a markedly different rate of decline of thermoregulatory processes in different individuals with ageing.^[5]

• Goose bumps
• Myoclonus
• Post micturition convulsion syndrome
• Chattering teeth
• Tremor

• Media related to Shivering at Wikimedia Commons
• Shivering at the U.S. National Library of Medicine Medical Subject Headings (MeSH)