Stress score

1

What does the Stress score show?

How your heart ran while you sat still, against your own usual for that hour.

On the Stress page, the top shows the day's figure with a word beside it and a sentence under them. The words are Calm, Steady, Raised and High. Home shows the same figure and word.

The figure averages your heart in the moments you sat still and awake. Each moment is set against your usual, your own still heart rate for that hour of the day. So a higher figure means your heart ran above its usual while you sat still. It does not say why.

2

What goes into it?

Your heart rate while the Watch says you were still, and now and then your heart rate variability.

Heart rate does most of the work, but only readings your Apple Watch marks as still: you were sitting or standing quietly. A reading without that mark counts as moving and stays out. Your heart rate variability, the small changes from one beat to the next, joins in when the Watch took a reading close to that moment.

Movement alone moves the heart, so researchers have tried to set it aside. In one study, 32 people wore a heart monitor for a day. A short calibration showed how each person's variability changed with movement. Stretches of 7.5 minutes or more where it fell further than movement explained were linked with worry, with tension and with lower positive mood, but not with how many stressful events people reported. Aera takes a simpler route and keeps only the moments the Watch calls still.¹

Sleep stays out, and so does the workout itself. A walk or a run lifts your heart because you are working. Stress also pauses after a workout: 90 minutes after an easy one, and 210 minutes after a harder one. A workout counts as harder when its average heart rate sits at least half way between your resting and your top heart rate. With no average heart rate, the easy pause applies.

Why pause at all? Wearable data from many thousands of runs showed a still heart rate raised for up to 180-210 minutes after a moderate or vigorous run, and not raised after a light one. Variability stayed lower for at least 300 minutes. Aera's two pauses are its own choice, set with that study in mind, and the longer one is shorter than the variability dip.²

3

How is a reading turned into a number?

Each still reading is scored against your usual for that hour, and the day's readings are averaged.

Your usual is the middle of your still heart rates over the 21 days before today, from 7 in the morning to 11 at night. Today never counts toward its own usual. The 21 days are Aera's choice, not taken from a study: over a shorter window, a tense or ill fortnight could become the usual.

A still heart beats a little faster at some hours than others. In that wearable study, still heart rate and variability both followed a daily rhythm, so the researchers compared each person's readings with the same time of day. Aera does the same: each hour gets its own small adjustment, learned from your last 30 days of still readings. A new watch starts from a default of 65 beats a minute until it has 30 still readings of yours.²

A swing is how far your still readings usually wander from your usual. On heart rate alone, a still reading at your usual for that hour scores 22, and each swing above it adds 18, up to 65.

Below your usual a reading falls more gently, 6 points a swing, to a floor of 5. A swing never counts as less than 3 beats a minute. Readings are smoothed.

Variability can change a moment when a reading sits within 5 minutes of it. It is judged against your overnight variability over the last 14 days, not against daytime readings. It mostly lowers a moment: a calm reading can take points off, and a poor one adds little. When both heart and variability read raised, they count equally. One review found average heart rate, which many smart devices use, less accurate for stress than variability.³

The day's number is the average of those moments. A moment with a variability reading beside it counts about twice as much as one from heart rate alone.

4

What do Calm, Steady, Raised and High mean?

Where your day's figure sits among your own recent days. Nobody else's days come into it.

The word sets the day's figure against your typical day over the last six weeks. Steady is close to your typical day. Calm is clearly below it, Raised clearly above it, and High well above it. Raised starts 0.75 spreads above the middle of those days, and High starts 1.5 spreads above it. A spread is how much your days differ from one another.

The figure and its word wait for your 7th scored day. Until then the page says Still learning and counts the days toward 7. Day one has no usual of its own yet, so nothing on it is marked.

On the day chart, still time is marked At your usual, Raised or Tense. Raised is one swing above your usual, a score of 40. Tense is a rise that holds: the smoothed line stays above 56 for at least 10 minutes.

The tense stretch then spans the whole time the line stayed above 48, and two rises closer than 15 minutes join into one.

The study above scored stretches of 7.5 minutes. Aera's 10-minute hold and its two lines are its own choices, set on real days of readings, not taken from a study.¹

5

When does it show nothing?

When too few still readings came in, the gear cannot tell when you sit still, or the day is behind Premium.

Today needs 10 still readings before it has a figure. Until then the page says Not yet and counts them for you. A day with fewer, or none, says that your watch recorded too few still moments, or no still moment at all.

Only an Apple Watch tells Aera when you sit still. With gear that cannot, the page says Apple Watch only and shows no number: a limit, not a wait. A watch worn only to work out says Workouts only.

Awake spells of 20 minutes or more with no heart reading are marked Watch off, so a gap in the chart reads as the watch, not as a calm. A day older than the last 30 has its figure behind Premium.

6

What can't it tell you?

Why your heart ran high. It sees the body's side of stress, and nothing of the cause.

A raised heart has many causes. In a smartwatch study of 3,318 people, built to catch infections early, alerts also came with stress, alcohol and travel, though less often than with infection. Aera cannot tell these apart either.⁴

When a day is raised or holds a tense stretch, the sentence under the figure can name one of two things Aera can see, set against your last 28 nights. One is a night more than 44 minutes under your usual sleep. The other is a sleeping heart rate at least 4 beats above its usual. Both need a week of earlier nights, and both cut-offs are Aera's choice, not taken from a study.

The Stress figure is not an input to Recovery or Room to train. Energy reads the same still readings, and more, for its own running level.

If stress feels heavy and does not lift, or you feel low or anxious, talk to a doctor.