How it works

Last updated: 2026-11-01

The Nervous System score is an estimate, from 0 to 100, of how much strain your body shows today compared with your own normal. It is a trend, not a medical measurement.

What it reads

Downshift reads six signals from Apple Health. Each one counts for a share of the score:

• Sleep, 25%. How long you were asleep. Nothing else about your sleep is scored.
• Resting heart rate, 24%.
• Breathing rate, 20%.
• Heart-rate variability (HRV), 20%.
• Sleeping wrist temperature, 7%. Apple Watch only.
• Activity, 4%. Active energy. Steps alone are not scored.

The weights are our own choice. No study sets them. Wearables measure all of these with limited precision. If a signal is missing, the shares of the others are scaled up to fill the gap.

Your own normal

Each signal is compared with your last 28 days, never with other people and never with a fixed number. Downshift uses the middle value and how much it usually varies, so one odd night barely moves your normal. A signal only counts once it has 14 days of readings behind it.

A higher resting heart rate, breathing rate or wrist temperature, a lower HRV, or less sleep than usual all point to more strain. Lower means less strain than usual.

Your normal is built from the days your watch or band recorded. If you switch to a different one, a reading such as HRV can come out higher or lower than before, because a new device may record it at different times of day. Your normal still holds the old readings, so it takes about four weeks to catch up. Until then, read that part of the score loosely.

Sensitive, not specific

These signals can shift when you are under stress, but they are not specific to it. A hard workout, a late drink, the start of a cold or a short night can move them in the same way. A higher reading does not always mean stress. Read it as a trend, and notice what else was going on that day.

Breathing rate and wrist temperature

Independent research on Apple Watch breathing rate and sleeping wrist temperature is more limited than for heart rate and steps; a published review of Apple Watch studies found less evidence for them. Downshift compares every signal only with your own normal, never with a fixed number, and treats wrist temperature as a small supporting cue.

What the score will not do

• Diagnose anything. It cannot tell you that you are ill, or that you are not.
• Judge you against a number. There are no healthy ranges and no targets, only your own past.
• Guess. It needs at least two of sleep, resting heart rate, breathing rate and HRV with 14 days of history. With fewer, it shows no number. With only activity data, such as steps or active energy, it shows none either.
• Nudge on one signal. The optional nudge to pause and breathe comes at most once a day, between 9:00 and 21:00. It needs at least two of those four signals to be clearly off your normal toward strain, for example a higher resting heart rate and a lower HRV, and either a high score or the same on two days in a row. A high score on its own is not enough.

Your data

The score is worked out on your device. The one thing that leaves it is an anonymous crash report, which holds no Apple Health values. See the privacy policy for the details.

Not a medical device

Downshift is a wellness and education tool, not a medical device. If your readings stay unusual, or you have symptoms that worry you, talk to a doctor or another qualified health professional. If you feel very unwell, contact your local emergency services. If you are in distress or thinking about harming yourself, contact your local emergency services or a crisis line in your country.

Contact

Questions: downshift@xymex.app