Late fall and winter can be a risky time to play outdoors, when a dangerous brew of frigid rain, icy winds, and near-freezing temperatures can quickly lead to hypothermia. As your core temperature begins to drop, your body unleashes a powerful heat-generating force: shivering. This spasmodic muscle activity is one of the most telling clues—and effective responses—to the possible onset of hypothermia.

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Read Hypothermia, Frostbite, and Other Cold Injuries.

FLEX AND RELEASE You start to shiver when your skin temperature, averaged over the entire body, cools below a certain point. This threshold varies from person to person but typically hovers between 80 and 85 degrees Fahrenheit, explains Dr. John Castellani, a research physiologist at the U.S. Army Research Institute of Environmental Medicine in Natick, Mass. “People need to pay attention to shivering because it is an early warning mechanism that the body is getting cold,” Castellani says.
Shivering begins before the body’s core temperature starts to drop and can be controlled voluntarily in its early stages. If you are able to stop shivering by thinking about it, then you are not yet hypothermic—though you may be headed that way. Shivering becomes increasingly intense as core temperature declines. By the time it hits 95 degrees, the upper limit of mild hypothermia, shivering has become involuntary. Complex tasks are difficult to perform with your hands, which have become numb as your body reduces blood flow to its periphery. Your body needs warmth to reverse the trend.

FEED THE FURNACE Your muscles generate a substantial amount of heat as they spasmodically contract and release. “A person at rest has a metabolic heat production of around 100 watts, like a bright lightbulb,” Castellani notes. “A person with a core temperature of 95 degrees and skin of 52 degrees can shiver at about four times the resting rate, or produce 400 watts.” This increased heat output can help stop, and potentially reverse, the descent into hypothermia. But in order to shiver, your body needs fuel. Carbohydrates provide the quickest energy uptake, though protein and fats provide energy over a longer period.

Vigorous exercise can also rapidly increase core temperature. “A person of average fitness exercising maximally can generate about 1,000 watts of heat,” Castellani says. If you’re stopped on the trail and shivering, start moving again to warm up, though be careful. Don’t sweat into your layers, which will increase your heat loss, and don’t do activities that shift clothing around and force warm air out, such as jumping jacks.

Most importantly, you should not let yourself become hypoglycemic (which means you have low blood sugar, or glucose). “Glucose helps the brain recognize that it is cold and better generate some heat,” Castellani continues. “People get hypoglycemic when they hike for long periods of time and eat very little. If there is no blood glucose, shivering will cease.” To quickly boost waning glucose levels, consume simple sugars—cookies, candy bars, fruit—then add more complex carbohydrates, proteins, and fats for prolonged energy. Avoid caffeine, nicotine, and alcohol, which all increase heat loss.
Also keep in mind that no matter how much you shiver, it won’t help if you can’t retain heat. Seek shelter from the wind and rain, remove wet layers and replace them with dry ones, and wrap yourself in as much insulation as possible.

THE FIRE DIMS As core temperature drops toward 90 degrees, shivering becomes increasingly violent. Walking and other gross motor skills become impaired. Confusion and sluggish mental activity are apparent. As your body descends below 90 degrees, the situation is life-threatening. Shivering becomes irregular and ceases at approximately 88 degrees. Warmth must now be supplied externally. Your best hope is that somebody wraps you in thick insulation and applies hot water bottles, heat packs, or chemical warmers to your neck, armpits, and groin. Otherwise, your core temperature will continue to decline and your chances for survival will rapidly dwindle.