Two metrics, one system
HRV is not a single number. Researchers use dozens of metrics, and the two you will meet most often are SDNN and RMSSD. They both come from the same heartbeat data, but they answer different questions.
SDNN: the whole-day picture
SDNN measures the standard deviation of all your normal heartbeat intervals over a period of time — often a full day. Because it sums up every influence your nervous system experienced, it behaves like a daily stress-and-recovery score: higher SDNN suggests a nervous system that is flexible and recovered; lower SDNN suggests accumulated load.
RMSSD: the moment-to-moment view
RMSSD measures the variation between successive heartbeat intervals. It is dominated by breath-linked, parasympathetic activity and responds quickly — ideal for spotting a response within seconds or minutes. Athletes often use RMSSD for morning readings or guided-session tracking.
Which one for daily stress tracking?
For following daily stress and recovery trends, SDNN has two advantages. First, it reflects the cumulative state of the whole day, so a morning single-point reading matters less. Second, it is the metric most consistently linked to overall stress load in research. RMSSD is better when you need a fast, repeatable snapshot in controlled conditions — the same time, same position, every morning.
Why Tuning tracks SDNN
Tuning is built around the daily question “what is my stress state today?” — which is exactly the question SDNN answers. By anchoring your SDNN to a personal 14-day rolling baseline, Tuning separates the signal (how your nervous system is doing) from the noise (what “normal” means for other people). RMSSD remains useful for acute moments; SDNN is the better long-range compass.
Key takeaway
SDNN is your daily weather system; RMSSD is the local radar. For understanding and improving your stress and recovery week over week, track SDNN — and compare it with yourself, not with strangers.