Health Metrics Glossary: What Your Wearable Data Actually Means
Your devices generate a lot of numbers, but the labels rarely explain what each one measures or how to read it. This glossary defines the consumer health-data terms you meet most often, in plain language, and points you toward reading each one against your own baseline rather than a population average.
Heart-rate variability (HRV)
Heart-rate variability is the variation in time between consecutive heartbeats, commonly used as an indirect window into your autonomic nervous system. Higher variability at rest generally reflects capacity to adapt; suppressed variability often accompanies hard training, poor sleep, alcohol, illness, or stress. HRV is intensely individual, so compare it to your own recent history, not published ranges. A guide on reading a low HRV reading walks through noise versus signal.
Resting heart rate (RHR)
Resting heart rate is your heart rate when you are fully at rest, usually captured overnight or on waking. A lower resting rate generally reflects better cardiovascular fitness and recovery, while a multi-day rise can accompany fatigue, illness, alcohol, or heat. What counts as normal varies widely between people, so the useful comparison is your own typical band, not a chart of population averages.
Readiness score
A readiness score is a composite estimate of how prepared your body is for exertion on a given day, blending inputs such as HRV, resting heart rate, sleep, and recent activity. Different apps compute it differently, so treat it as a directional summary rather than a precise verdict. Its value comes from the trend and from checking which underlying signals moved, not the headline number alone.
Recovery score
A recovery score estimates how well your body has bounced back from recent stress and training, typically drawing on overnight HRV, resting heart rate, and sleep quality. Like readiness, it is a composite that varies by vendor. Read it as a rolling summary of your recent state rather than an absolute grade, and look underneath it when it shifts to see which inputs are actually driving the change.
Strain
Strain is an estimate of the cardiovascular load you have accumulated across a day, usually derived from heart rate spent in different intensity zones. It reflects how much stress you have placed on your system, not calories or step count. Because your capacity changes with fitness and recovery, the same activity can register as more or less strain over time; read it against your own recent days.
Sleep debt
Sleep debt is the running gap between the sleep you have gotten and the sleep your body appears to need, accumulated over recent nights. It reframes short nights as a ledger you can pay down rather than a single failure. The most useful reading is the multi-day trend: one short night matters little, while a debt that keeps growing across a week is worth addressing with earlier nights.
Deep sleep
Deep sleep, also called slow-wave sleep, is the stage associated with physical restoration, when the body does much of its repair work. It typically occurs more in the first half of the night and declines with age. Consumer devices estimate it rather than measure it directly, so treat the figure as approximate. Watch your own night-to-night pattern instead of chasing a fixed target from population data.
REM sleep
REM (rapid eye movement) sleep is the stage associated with dreaming and with memory and emotional processing, marked by heightened brain activity. It tends to cluster in the later part of the night, so cut-short mornings can trim it. Wearables estimate REM from movement and heart-rate patterns, so read it as an approximation. Your own baseline proportion is a better reference than any single recommended number.
Respiratory rate
Respiratory rate is the number of breaths you take per minute, usually measured while you sleep. It tends to be stable night to night for a given person, which is what makes it useful: a sustained rise above your normal can accompany illness, heat, alcohol, or stress. Because individual baselines differ, a reading that looks high in isolation is best judged against your own typical overnight value.
VO2 max
VO2 max is an estimate of the maximum amount of oxygen your body can use during intense exercise, a common proxy for cardiorespiratory fitness. Wearables estimate it from heart rate and pace rather than lab testing, so the exact figure is approximate. Its trend over weeks and months is more informative than any one reading, and improvements generally reflect training that is building aerobic capacity.
Personal baseline
A personal baseline is what is normal for you on a given metric, learned from your own recent history rather than a population range. It is the reference that makes every other number meaningful, because healthy values span an enormous spread between individuals. A reading only becomes a signal when it moves clearly outside your own band. See what a personal health baseline is for more.
Autonomic nervous system
The autonomic nervous system regulates involuntary functions such as heart rate, breathing, and digestion, balancing "fight or flight" and "rest and digest" activity. Wearables cannot observe it directly; they infer its state from signals like HRV and resting heart rate. This is why those metrics are read as indirect windows rather than exact measurements, and why patterns across several days tell you more than any single number.
Skin temperature deviation
Skin temperature deviation is how far your overnight skin temperature sits from your own recent average, rather than an absolute reading in degrees. Framing it as a deviation is deliberate, because normal skin temperature varies by person and environment. A sustained shift can accompany illness, hormonal cycles, alcohol, or a warm room, and it is most informative when other signals agree with it.
Building your health data into one model that learns these baselines for you is what we built NuVARD to do.
NuVARD provides wellness intelligence. It does not diagnose, treat, or replace medical care.