Heat and cold hormesis: the science behind sauna and cold plunge

Hormesis is the principle that low doses of a stressor that would be harmful at high doses can produce beneficial adaptive responses. The concept derives from toxicology but has been extended to exercise, caloric restriction, heat, cold, and even certain phytochemicals. Heat and cold are the two most studied hormetic stressors in the longevity context, and they produce distinct but complementary physiological adaptations. The evidence base for heat (sauna) is substantially stronger than for cold exposure, but both have compelling mechanistic rationales.

The sauna cardiovascular data

The Laukkanen et al. (JAMA Internal Medicine, 2015) study is the cornerstone of the sauna longevity literature. It followed 2,315 Finnish men for 20 years and found a striking dose-response relationship between sauna frequency and cardiovascular mortality. Men using the sauna 2–3 times per week had a 22% lower risk of cardiovascular mortality compared to once-weekly users. Men using it 4–7 times per week had a 40% lower risk. The relationship persisted after adjustment for conventional cardiovascular risk factors including blood pressure, cholesterol, smoking, and physical activity.

A follow-up study by the same group (Age and Ageing, 2018) found that frequent sauna use (4–7 times per week) was associated with a 66% lower risk of dementia and a 65% lower risk of Alzheimer's disease compared to once-weekly use. These are extraordinary effect sizes for a passive lifestyle intervention. The mechanisms are not fully established, but likely include improved cerebrovascular function, reduced inflammation, and heat shock protein-mediated neuroprotection.

Heat shock proteins: the molecular mechanism

The primary molecular mechanism of sauna's benefits is the induction of heat shock proteins (HSPs), particularly HSP70 and HSP90. HSPs are molecular chaperones that assist in the proper folding of proteins and the degradation of misfolded proteins. They are upregulated in response to heat stress and protect cells from the damage caused by protein aggregation — a hallmark of aging and neurodegenerative disease.

A single sauna session at 80–90°C for 30 minutes produces a 2–3-fold increase in HSP70 expression in peripheral blood mononuclear cells. Regular sauna use produces a sustained upregulation of HSP expression, improving the cell's capacity to handle protein stress. This is directly relevant to the proteostasis hallmark of aging: the accumulation of misfolded proteins (amyloid-β, tau, α-synuclein) in neurodegenerative disease is driven by failure of the proteostasis machinery, and HSP induction may slow this accumulation.

Cardiovascular adaptations to heat

A sauna session at 73–82°C produces a cardiovascular response similar to moderate aerobic exercise: heart rate increases to 100–150 bpm, cardiac output doubles, and peripheral vasodilation occurs as the body attempts to dissipate heat. This cardiovascular 'workout' without musculoskeletal loading may be particularly valuable for people who cannot exercise due to injury or disability.

Regular sauna use improves arterial compliance (reducing arterial stiffness), lowers resting blood pressure, and improves endothelial function — all established cardiovascular risk factors. The mechanism involves nitric oxide (NO) production: heat stress activates endothelial nitric oxide synthase (eNOS), increasing NO production and driving vasodilation. Regular eNOS activation through sauna use may maintain endothelial function as a form of vascular conditioning.

Cold exposure: the evidence

The evidence base for cold exposure (cold water immersion, cold showers, cryotherapy) is substantially weaker than for sauna. There are no large prospective cohort studies showing that cold exposure reduces mortality. The evidence is primarily mechanistic and from small, short-duration trials. This does not mean cold exposure is without benefit — the mechanistic rationale is compelling — but it means the claims made in popular culture substantially outpace the data.

Brown adipose tissue and thermogenesis

Cold exposure activates brown adipose tissue (BAT) — a specialised fat tissue that generates heat by uncoupling the mitochondrial electron transport chain (via UCP1, uncoupling protein 1). BAT activation burns glucose and fatty acids without producing ATP, generating heat instead. Adults have variable amounts of BAT, concentrated in the supraclavicular and paravertebral regions. Regular cold exposure increases BAT volume and activity, improving cold tolerance and potentially improving metabolic health.

The metabolic benefits of BAT activation are real but modest in magnitude. A maximally stimulated BAT depot in a healthy adult burns approximately 200–300 kcal per day — meaningful but not transformative. The more important benefit may be the improvement in insulin sensitivity and glucose metabolism associated with BAT activation, which is mediated by BAT-secreted factors including FGF21 and adiponectin.

Norepinephrine and the cold shock response

Cold water immersion produces a rapid and substantial increase in norepinephrine (noradrenaline) — up to 300% above baseline within minutes of immersion. Norepinephrine is both a neurotransmitter and a hormone with wide-ranging effects: it improves focus and alertness, reduces inflammation (via β2-adrenergic receptor signalling), and activates the sympathetic nervous system. The acute norepinephrine surge from cold exposure is one of the primary mechanisms proposed for its mood-elevating and anti-inflammatory effects.

Cold exposure also activates the vagus nerve and may improve heart rate variability (HRV) with regular practice. A 2021 study in PLOS ONE found that 30 days of cold shower exposure (ending with 30–90 seconds of cold water) reduced self-reported sick days by 29% compared to controls, though the mechanism is unclear.

The critical caveat: cold blunts hypertrophy

Cold water immersion immediately after resistance training significantly blunts the hypertrophic adaptation to training. Roberts et al. (Journal of Physiology, 2015) found that cold water immersion after resistance training attenuated muscle hypertrophy and strength gains over 12 weeks compared to active recovery. The mechanism: cold exposure suppresses the mTORC1 signalling and satellite cell activation that drive muscle protein synthesis in the hours after training. If building or maintaining muscle mass is a priority — and for longevity, it should be — cold immersion should be separated from resistance training by at least 4–6 hours, or reserved for non-training days.

Combining heat and cold

The popular practice of alternating between sauna and cold plunge (contrast therapy) has a long tradition in Scandinavian and Eastern European cultures. The physiological rationale is that the alternating vasodilation (heat) and vasoconstriction (cold) produces a 'vascular pump' effect that may improve circulation and recovery. The evidence for this specific practice is limited, but it appears safe for healthy adults and may enhance the subjective recovery experience.

The practical protocol

• Sauna: target 4–7 sessions per week at 73–82°C for 15–20 minutes per session. This matches the frequency associated with the lowest cardiovascular mortality in the Laukkanen data.
• If 4–7 sessions per week is not feasible, 2–3 sessions per week still produces a 22% cardiovascular mortality reduction — a meaningful benefit.
• Stay well hydrated: drink 500 ml of water before and after each sauna session. Avoid alcohol before or during sauna use.
• Cold exposure: start with ending your shower with 30–60 seconds of cold water. Progress to 2–3 minutes of cold water at the end of each shower over 4–6 weeks.
• Cold water immersion (10–15°C for 5–10 minutes) produces stronger physiological responses than cold showers. Use on non-training days or at least 4–6 hours after resistance training.
• Do not use cold immersion immediately after resistance training: it blunts hypertrophic adaptation.
• Contraindications for sauna: cardiovascular disease, uncontrolled hypertension, pregnancy, recent alcohol consumption. Consult your physician if you have any cardiovascular conditions.