When your body’s core temperature drops below 95°F (35°C), hypothermia begins. Your body launches an aggressive defense to protect your vital organs, but as the cooling continues, those defenses fail in a predictable sequence. What starts as shivering and confusion can progress to dangerous heart rhythms, bizarre behavior, and eventually cardiac arrest.
How Your Body Fights the Cold
Your autonomic nervous system, the same system that controls your heart rate and breathing, runs your body’s thermostat. The moment cold exposure starts pulling your core temperature down, two main defenses kick in.
First, blood vessels near your skin’s surface constrict, pulling warm blood away from the surface and toward your core. This is why your fingers, toes, and nose go pale and numb first. Your body is essentially sacrificing the extremities to keep your heart, lungs, and brain warm. Second, you start shivering. Shivering is involuntary rapid muscle contraction, and it can increase your heat production several times over normal levels. It’s effective but exhausting, and it burns through your energy reserves fast.
These two mechanisms work well in mild hypothermia, roughly between 90°F and 95°F. But they have limits. Once your core temperature drops below about 90°F, shivering typically stops. That’s not a sign of improvement. It means your body has run out of energy to generate heat, or your muscles are too cold to contract properly. From this point, cooling accelerates.
What Happens to Your Heart
Your heart is particularly vulnerable to cold. As your core temperature falls below about 96°F, your heart rate starts slowing. By the time you reach moderate hypothermia (below 89.6°F or 32°C), the electrical signals controlling your heartbeat become erratic. The intervals between beats stretch out, and the heart’s electrical patterns develop characteristic abnormalities that doctors can see on a heart monitor.
Below 82°F (28°C), the risk of a fatal heart rhythm called ventricular fibrillation rises sharply. In ventricular fibrillation, the heart quivers chaotically instead of pumping blood. What makes this especially dangerous is that a cold heart resists treatment. The standard drugs used to correct abnormal rhythms become less effective, and defibrillation (the electrical shock used to restart normal rhythm) is harder to accomplish when the heart muscle is that cold.
Changes in Your Blood and Kidneys
Cold does something counterintuitive to your kidneys: it makes you urinate more. This phenomenon, called cold diuresis, happens because the blood vessels near your skin constrict and push blood volume toward your core, temporarily raising blood pressure in your central organs. Your kidneys respond to this apparent excess by filtering out more fluid. The result is that you lose water volume at exactly the wrong time.
As you lose fluid and your body cools further, your blood thickens. Research from Johns Hopkins has identified a critical temperature zone between roughly 50°F and 68°F (10°C to 20°C) where blood viscosity increases disproportionately. Thicker blood moves through tiny capillaries with more difficulty, reducing oxygen delivery to tissues. This combination of dehydration, thickened blood, and impaired circulation compounds the strain on your heart and brain.
How Your Brain and Behavior Change
Confusion is one of the earliest and most recognizable signs of hypothermia. As your brain cools, your thinking slows, your speech slurs, and your coordination deteriorates. People in moderate hypothermia often make poor decisions, like failing to seek shelter or being unable to zip a jacket. This impaired judgment is part of what makes hypothermia so dangerous: by the time you really need to help yourself, you may no longer be capable of it.
In severe and terminal hypothermia, two strange behaviors can occur. The first is paradoxical undressing, where a person removes their clothing despite freezing temperatures. This happens because the blood vessels near the skin, which have been tightly constricted to conserve heat, suddenly dilate. The rush of warm blood to the skin creates an overwhelming sensation of heat. People in this state feel like they’re burning up and strip off their clothes, which only accelerates heat loss.
The second is terminal burrowing. People in the final stages of hypothermia often crawl into small, enclosed spaces, wedging themselves under beds, behind furniture, or into closets. Researchers at the Icahn School of Medicine at Mount Sinai have described this as a primitive, autonomous response from the brain stem, similar to the burrowing behavior of hibernating animals. It appears to be a last-ditch protective instinct. Bodies found after fatal hypothermia are frequently discovered in these hidden, cramped positions, which can initially look suspicious to investigators unfamiliar with the phenomenon.
The Stages in Sequence
Hypothermia progresses through three recognized stages:
- Mild (above 93.2°F / 34°C): Shivering, numbness in extremities, difficulty with fine motor tasks like buttoning a coat. You’re alert but your coordination and judgment are slipping.
- Moderate (86°F to 93.2°F / 30°C to 34°C): Shivering becomes violent, then may stop. Confusion worsens, speech becomes slurred, drowsiness sets in. Heart rate slows and becomes irregular.
- Severe (below 86°F / 30°C): Shivering stops entirely. The person may be semiconscious or unconscious. Heart rhythms become dangerously erratic. Paradoxical undressing and terminal burrowing may occur. Cardiac arrest becomes likely.
Why Rewarming Is Tricky
Getting warm again isn’t as simple as it sounds. One of the most important concepts in hypothermia treatment is “afterdrop,” where your core temperature actually continues to fall even after rewarming has started. This happens because of basic physics: the outer layers of your body are colder than the core, and heat naturally flows from warmer areas to cooler ones. When you begin warming the surface, the deeper tissues are still losing heat outward to the colder layers around them. It takes time for the warming to penetrate inward, and during that delay, core temperature keeps dropping.
Afterdrop is dangerous because it can push a moderately hypothermic heart into ventricular fibrillation. This is why rewarming methods are matched to severity. For mild hypothermia, passive rewarming works: warm blankets, a heated room, removing wet clothing, and letting the body’s own heat production do the rest. For moderate hypothermia, active external warming is used, including heated blankets, forced warm air devices, and warmed fluids. Severe hypothermia with cardiac arrest requires invasive internal warming techniques, where heat is delivered directly to the core through methods like warmed fluid circulated through the chest cavity or, in some cases, machines that warm the blood outside the body and return it.
Rapid rewarming of the extremities before the core is warm can also cause problems. When cold, constricted blood vessels in the arms and legs suddenly open, they can dump cold, acidic blood back into the heart, triggering arrhythmias. This is why the general principle is to warm the trunk first and handle the limbs gently.
Who Is Most Vulnerable
Hypothermia doesn’t require extreme cold. It can develop at temperatures well above freezing, especially in wet or windy conditions. Water conducts heat away from your body roughly 25 times faster than air, so falling into cold water or wearing wet clothing dramatically accelerates cooling.
Older adults are at higher risk because their thermoregulatory responses weaken with age. They may not shiver as effectively or notice cold as readily. Very young children are vulnerable because of their high surface-area-to-body-mass ratio, which means they lose heat faster. Alcohol increases risk by dilating blood vessels near the skin, which feels warm but actually dumps core heat into the environment. People who are exhausted, malnourished, or have certain medical conditions that impair circulation are also at greater risk.
One critical point about severe hypothermia: people can appear dead but still be resuscitable. Heart rate and breathing can slow to nearly undetectable levels. The medical principle is that a hypothermic person is not considered dead until they are “warm and dead,” meaning rewarming has been attempted and failed. There are documented cases of people surviving core temperatures below 60°F (15.5°C) with aggressive rewarming and prolonged resuscitation.