HRV (Heart Rate Variability)

Main metrics measured by overnight continuous monitoring (wearables like Whoop, Oura, Apple Watch, Garmin):

RMSSD (Root Mean Square of Successive Differences): the most-used pure vagal metric. Typical values: 30-60 ms in young adults, progressive decline with age. SDNN: total variability over 24 h. Value <70 ms in people over 50 predicts higher cardiovascular mortality. LF/HF ratio: sympathetic/parasympathetic balance. pNN50: percentage of consecutive intervals differing >50 ms.

Factors that increase HRV: restorative sleep, regular Zone 2 aerobic exercise, mindfulness meditation, conscious slow breathing (4-6 bpm), adequate hydration, nature exposure, social connection, controlled fasting.

Factors that lower HRV: chronic stress, insufficient sleep, alcohol (acute and chronic), late heavy meals, dehydration, overtraining, infection, chronic inflammation, depression.

Prognostic value: low HRV predicts major cardiovascular events, all-cause mortality, and cognitive decline. Vagus nerve stimulation (transcutaneous or invasive) raises HRV and is being investigated for multiple inflammatory and mental health pathologies.