When Your Body Is Under Stress Overnight

Most readiness scores answer one question: how did I do last night? That is useful, but it misses two things. First, a single signal can move for boring reasons: a warm room, a late meal. Second, a bad night is different from a bad week. Metrya has two views for this: Sentinel for "is tonight unusual?" and Recovery debt for "how long has this been going on?"

Sentinel: Five Overnight Signals, Read Together

Each morning, Sentinel compares last night with your own normal from the previous 28 nights. It uses five signals, each with a weight:

Each signal is scored by how far it is from your median, using a robust measure of spread so one strange night in your history doesn't distort the baseline. The five scores are combined into a 0–100 Body Stress Index. If a signal is missing (for example an older watch without wrist temperature), its weight is shared among the others.

Clear, Watch, Alert

The default setting is conservative, so Watch and Alert need a clear signal. To go back to Clear, the index has to fall well below the Watch level, which stops the state from flickering between nights. Sentinel needs at least 10 nights of a signal before it trusts that signal's baseline.

Sentinel and your Recovery score

When Sentinel sees a clear overnight problem, it can cap your Recovery score, and the Recovery sheet says so, for example "Held at 75 by a raised temperature". That stops a morning with great HRV but a fever-like temperature from telling you to train hard.

Recovery Debt: How Long, and How Fast Do You Bounce Back?

Sentinel looks at one night. Recovery debt looks at the run of days. Each day it adds up how far your signals were outside your normal range: the five Sentinel signals, plus sleep timing and training load.

The useful part is the history. Metrya reads up to a year of nights from Apple Health to find your past runs. So when a new run starts, you can compare it with earlier ones: how long did they last, and how fast did you get back to normal? You can also log illness, a vaccine, alcohol, travel or hard training on the same screen, and these tags appear next to your runs.

What Recovery debt doesn't do

• It doesn't change Recovery or Reserve. Sentinel already accounts for the overnight signals, and counting them again would punish one night twice.
• It never sends notifications. When a run is open, the Sentinel row on Today shows the balance and the day of the run.
• It shows logged events next to runs, not as causes.

How This Differs from the Stress Monitor

The Stress Monitor reads daytime HRV readings one at a time. Sentinel and Recovery debt read whole nights and several signals. They stay separate, so you get three distinct views instead of one blended score that hides where a change came from.

What You Need

• An Apple Watch or another wearable that writes overnight heart rate, HRV and sleep to Apple Health. Wrist temperature needs Apple Watch Series 8 / Ultra or newer, or a device like Oura.
• 10 nights for Sentinel to learn each signal. Recovery debt needs those 10 nights, then a week of data, and Sentinel must be on.

Limits

• It can't detect illness. A raised index means your body is under some strain. It could be a virus, a hard week of training, alcohol, heat, jet lag or a new medication.
• It can miss things. Some illnesses change none of these signals, especially early.
• Wearing matters. A loose watch or a missing night weakens the picture. Missing signals are shown as missing, with the reason.
• Feel unwell? Trust that first. If you have symptoms, follow medical advice, whatever the app says.