Cortisol and HRV: How Your Stress Hormones Shape Heart Rate Variability
Cortisol is the slow, long-acting arm of your stress response, and it tends to move opposite to vagal HRV: when cortisol is high, beat-to-beat variability (RMSSD, HF power) usually runs low. That is why HRV is naturally lowest in the first hour after waking, when the cortisol awakening response peaks. Cortisol does not appear in an HRV reading directly, so use HRV as a same-time-of-day trend and read the two together, not as a cortisol test.
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