Cold receptors on the skin signal the hypothalamus to increase neurotransmitters like norepinephrine, adrenaline, and cortisol, leading to brown fat activation.

“That's non-negotiable because it's mediated by cold receptors on the surface of your body and your skin and the way that they trigger the release of norepinephrine and epinephrine, not just from the adrenals, from the adrenal glands above your kidneys, but also from regions of your brain, like the locus coeruleus, which cause increases in the tension and alertness, and from other locations in your body where epinephrine and norepinephrine are released.”