Step into a sauna. Heat envelops you. Heart rate surges. Then, something remarkable happens later that night. Resting heart rate plummets. Terra’s analysis of 59,000 days from 256 wearable users shows nighttime heart rates drop about 3 beats per minute on sauna days—a 5% decline versus non-sauna days. This holds even after adjusting for elevated activity levels on those same days. Terra Research calls it a true physiological recovery signal. Sauna days aren’t lazy ones. Users log more activity time, cover greater distances. Maximum and average heart rates climb higher. Minimum heart rates dip lower. Women ramp up activity more than men but see a subtler nighttime drop. Statistical rigor backs the findings: FDR-corrected p-values under 0.05, Cohen’s d exceeding 0.2. And for women? Menstrual cycle phase shifts the equation. The drop intensifies in the luteal phase, with effect sizes pushing Cohen’s d above 0.2. Follicular phase? Weaker signal.…