An Apple Watch collects a startling amount of data about you and then presents most of it as a chart with no opinion attached. You are shown that your heart rate variability was 34 milliseconds last night. Whether that is good, bad, or entirely unremarkable is left as an exercise for the reader.
Here is the plain version for the four numbers that carry most of the signal.
Resting heart rate
The most useful number on the watch, and the one people ignore in favour of flashier metrics.
It is roughly how many times your heart beats per minute when you are doing nothing. Typical adults sit somewhere between 55 and 75. Endurance athletes run lower, sometimes in the 40s.
The absolute figure matters far less than your own trend. A resting heart rate that has drifted up five beats over a fortnight is telling you something — the usual candidates are a virus arriving, poor sleep accumulating, alcohol, heat, or training harder than you are recovering from. Sometimes it moves before you feel anything, which is the genuinely useful part.
Watch the direction, not the number.
Heart rate variability
The most misunderstood metric in consumer health, largely because people compare it to other people.
HRV measures the variation in time between consecutive heartbeats. A healthy heart does not tick like a metronome; the gaps flex slightly with your breathing and your autonomic nervous system. More variation generally indicates a body in a recovered, parasympathetic state. Less indicates stress, of whatever kind.
Two things make it awkward.
The between-person spread is enormous. One perfectly healthy adult sits at 20 milliseconds, another at 150. Comparing your figure to a friend's, or to a chart of population averages, tells you close to nothing. It is heavily influenced by age and by genetics.
And it is noisy. Night-to-night swings of 20 or 30 per cent are ordinary. A single low reading is not a finding.
Used properly it is a seven-day rolling average compared against your own baseline. Alcohol reliably suppresses it, usually for a night or two. So do illness, late meals, hard training, and stress that has nothing to do with your body at all — a bad week at work shows up here.
Sleep
Your watch estimates sleep stages from movement and heart rate. It is an estimate, and the stage breakdown is the least reliable part. Nobody should be optimising their deep sleep percentage from a wrist sensor.
What it measures well is duration and consistency, and consistency is the one that matters. Going to bed and getting up at roughly the same time does more for how you feel than chasing an extra fifteen minutes of REM. If the app tells you one thing about your sleep, it should be the variance in your bedtime.
Cardio fitness
Apple's VO2 max estimate, inferred from heart rate during brisk walks and runs. As an absolute figure it is approximate. As a trend over months it is one of the better long-range health indicators available on a consumer device, and it responds to training slowly enough that week-to-week changes are noise.
Reading them together
Individually each number is weak. Together they form a picture.
Resting heart rate up, HRV down, sleep short, and you are under load. That may be a cold coming, or a heavy training block, or a genuinely difficult month. The body does not distinguish between kinds of stress as neatly as we do.
Resting heart rate steady, HRV at baseline, sleep consistent, and you are fine, whatever the individual charts happen to show today.
The mistake nearly everyone makes is reacting to single readings. One bad night is a bad night. A fortnight of drift is information.
Where the app comes in
Pulse reads what is already in Apple Health and writes a short daily summary of it in ordinary language, rather than adding another dashboard to the pile.
The summarising is done by Apple's on-device language model, which matters here more than it does elsewhere: health data is about as personal as data gets, and none of it leaves the phone. There are no servers, no account and no analytics. Trends run across sleep, resting heart rate, active energy and HRV, and there is an optional one-off purchase for longer history.
If something in your numbers worries you, see a doctor rather than an app. A wrist sensor is a decent indicator of trend and a poor diagnostician.