Sitting in water near 40°C raises inner body temperature in a pattern resembling moderate exercise. Strongest findings: lower blood pressure, better artery flexibility, faster and better sleep, less pain in worn knees, though benefit size is genuinely uncertain. Harms are real and mostly avoidable: fainting on standing, drowning in older bathers, temporary loss of sperm quality, early-pregnancy risk. (Full Review)
| Marker | Target | Why |
|---|---|---|
| Home seated blood pressure | 105–120 / 65–78 mmHg | Primary benefit and acute risk |
| Resting heart rate | 50–65 beats/min | Tracks cardiovascular adaptation |
| Fasting glucose | 75–86 mg/dL (4.2–4.8 mmol/L) | Detects fasting improvement |
| Fasting insulin | 2–5 µIU/mL | More sensitive than glucose in trials |
| HbA1c | 4.8–5.3% | Confirms whether fasting gains persist |
| hs-CRP | <0.8 mg/L | Tests heat effect on inflammatory tone |
| Serum sodium | 137–142 mmol/L | Detects sodium depletion or over-drinking |
| Haematocrit | 38–46% women, 41–50% men | Flags plasma volume contraction |
| Body mass change across one session | Loss under 1% of body mass | Measures fluid replacement adequacy |
| Total motile sperm count (men attempting conception) | Above 20 million | Detects the reversible fertility effect |
Cadence: Baseline over at least one week. Blood pressure weekly for a month, then monthly; glucose, insulin, HbA1c, hs-CRP and sodium at 8–12 weeks, then every 6–12 months; body mass monthly.