Glossary

The variation in time between one heartbeat and the next, measured in milliseconds. It shifts with sleep, stress, illness, alcohol and training, so recovery apps compare today's value with your own baseline rather than with other people.

Normal HRV by age →

The standard deviation of the intervals between normal heartbeats — a measure of overall HRV. It is the HRV measure Apple Watch records in Apple Health, so compare Apple Watch readings with SDNN norms, not RMSSD ones.

SDNN norms by age and sex →

The root mean square of successive differences between heartbeats — a short-term HRV measure that mainly reflects the parasympathetic (rest-and-digest) nervous system. Some wearables report RMSSD instead of SDNN, and the two numbers are not interchangeable.

RMSSD norms by age and sex →

How many times your heart beats per minute when you are fully at rest. For adults, the American Heart Association gives a normal range of 60 to 100 bpm; fitness, age, medication and other factors move it.

Normal resting heart rate by age →

An easy, steady aerobic intensity at which you can still hold a conversation. On this site Zone 2 means 65–75% of maximum heart rate, with max heart rate estimated by the Tanaka formula; other sources draw the zone slightly differently.

Zone 2 heart rate calculator →

A single daily number, usually 0–100, that combines signals such as HRV, sleep, resting heart rate and recent training load against your own baseline to suggest whether to train hard or recover. Each app weights the inputs differently, so scores from different apps are not comparable.

Readiness score calculator →

A running total of how much training stress you have done, usually built from how long and how hard each workout was. Apps compare recent load with your longer-term average to show whether you are building gradually or ramping up too fast.

Track training load in Body Vitals →

Your last 7 days of training load divided by your average weekly load over the last 28 days. Values of 0.80–1.30 are the commonly cited lower-risk range and above 1.50 is treated as a spike, though the thresholds are debated.

ACWR calculator →

The maximum amount of oxygen your body can use during exercise, in millilitres per kilogram per minute — a core measure of cardiorespiratory fitness. Apple Watch estimates it as Cardio Fitness during outdoor walks, runs and hikes.

See VO2 max in Body Vitals →

The different kinds of sleep you cycle through each night. Apple Watch reports time Awake and in REM, Core and Deep sleep, which you can view in the Health app.

Sleep stages widgets in Body Vitals →

The color beneath the surface of your skin — warm (golden or peachy), cool (pink or bluish) or neutral. Color analysis uses undertone to decide whether warm or cool shades flatter you; it is a styling idea, not a medical measure.

Find your undertone in the color season quiz →

A styling system that groups natural coloring into Spring, Summer, Autumn and Winter — split into 12 seasons by warmth, depth and clarity — and suggests a palette for each. It is a style guide, not science.

Color season calculator →