Key Takeaways

  • The sharpest decline in your core temperature occurs after you step out of the cold, a phenomenon metabolic scientist Dr. Susanna Søberg identifies as the after-drop.
  • During cold immersion, blood vessels constrict to keep warm blood centered around vital organs; once you exit, those peripheral vessels dilate, sending blood across chilled surface tissue where it cools before returning to the core.
  • Dr. Søberg compares shivering to physical training at a gym because involuntary muscle contractions create cellular hormesis and activate brown adipose tissue.
  • Allowing yourself to shiver naturally after cold exposure upregulates shock proteins and drives metabolic activation instead of cutting the physiological work short.

The blood flow mechanics behind the after-drop

When you submerge yourself in cold water, your body enters defense mode. Blood vessels and capillaries near the surface constrict immediately. This vasoconstriction pulls warm blood away from your skin and shunts it deep into your center to protect your brain, heart, and vital organs. You feel freezing, but your core temperature remains shielded during the initial plunge.

The real plunge in core temperature begins the moment you step onto dry land. As your nervous system senses that the direct cold source is gone, those constricted peripheral vessels open up again. As Søberg explains: “So as soon as you get up that those blood vessel will open again and the warm blood would flow out and get colder and then flow back again into the core and that's going to decrease the temperature in your core.” (play from 8:22)

Huberman summarizes this delayed reaction plainly. Peripheral vessels constrict to keep warm blood near the center of the body. Once you get out, capillaries dilate again. The blood travels to the icy skin, dumps its heat, and cycles straight back into your core. Søberg calls this the after-drop. You step out of an ice bath feeling tough, and two minutes later your teeth chatter violently. That lag is not a mistake. It is cold blood completing the circuit back to your heart.

Why shivering works like resistance training for cells

Most people treat shivering as an uncomfortable nuisance to suppress. They reach for a heavy parka, blast the car heater, or hop straight into a hot shower. Søberg argues that doing this shuts down the exact biological response you got into the cold to trigger.

Shivering is involuntary, high-frequency muscular contraction. Søberg compares this biological reaction to lifting weights: “If you can like in your mind get used to the thought of shivering is just like when you go exercising in the training center and get that feeling of like oh this is tough now it hurts a little bit. Yeah it's going to hurt because that's what shivering also does but it's just a different way of training your cells in your body.” (play from 9:25)

That discomfort triggers hormesis, which Søberg defines as healthy cellular stress. When cold-induced shivering forces your muscle cells and brown adipose tissue to work to restore thermal balance, it upregulates protective shock proteins and sharpens metabolic activation. Brown fat cells, which burn energy directly to generate heat, become better at activating under stress. If you numb the cold shock with immediate external warmth, you cut short the muscular and brown fat stimulation before your cells finish the work.

What to Do With This

When you finish cold immersion, resist the reflex to jump into a hot shower or put on a heavy coat. Stand still, let the after-drop initiate, and let your body shiver naturally for three to five minutes so your muscles and brown fat do the work of rewarming your core.