Resting heart rate is the heart rate measured immediately upon waking, before standing or stimulus. Reflects baseline autonomic tone.
Mechanism — Lower RHR generally reflects greater cardiac efficiency and parasympathetic dominance. Trained endurance athletes hit 40-50 bpm; well-trained adults 50-60; sedentary 70+. Acute elevations of 5-10 bpm from baseline signal stress, infection, alcohol, poor sleep, or overreaching.
Use case — Daily morning check via wearable or manual count. Track trend over weeks. Spike from baseline = back off training, prioritize recovery.
Caveats — Individual baselines vary widely. A 'high' 70 for one person is a normal 70 for another. Genetics, medications (beta-blockers), and caffeine timing all skew it.