Seven signals, or one?
For a while the story looked like a chorus: half a dozen different signals on the watch all pointing the same way before a crash. Independent witnesses agreeing is powerful evidence. But when we checked whether they were actually independent, the chorus turned out to be mostly the same voice, recorded several ways.
The reduction
These are seven measured autonomic channels, some load-and-recovery, some orthostatic, and they don't measure seven different things. Grouped by how strongly they move together, they collapse into 3 roughly independent clusters:
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Within-day stress
Stress spikesStress spikes (window variant)Within-day U-dips
H02b and H02d are the very same daily number measured twice; they correlate 1.00. HA11 shares about 0.38.
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Overnight autonomic state
Overnight stress levelMorning body batteryMorning heart-rate swing
Morning body-battery and overnight stress are the same signal in mirror image (−0.92, because Garmin builds one from the other). The heart-rate swing shares about 0.38.
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Autonomic variability
Overnight stress variability
Related to the autonomic-state cluster but not collinear (about 0.50): one reads the level of the overnight signal, this one reads its movement. The variability-collapse picks out crash-weeks that the level reading misses.
The ties that collapse them
Each of these is a measured correlation. A value near +1.00 or −1.00 means two "separate" signals are really one (the minus sign just means one rises as the other falls):
- +1.00 Stress spikes & Stress spikes (window variant) the identical daily primitive, measured twice
- -0.92 Overnight stress level & Morning body battery the same signal, mirror image
- +0.50 Overnight stress level & Overnight stress variability the level and the movement of one channel
- +0.38 Morning heart-rate swing & Overnight stress level within the autonomic-state cluster
- +0.38 Stress spikes & Within-day U-dips within-day stress, shared variance
What it does to the headline
Counting honestly (three or four independent clusters, not seven separate tests) only 1 clears conventional statistical significance at this sample size. Corrected honestly for roughly three independent clusters rather than seven separate tests, one cluster (the within-day stress spike) is the only one that clears the conventional one-in-twenty line in this multiplicity check. But be exact about what that does and doesn't mean. Judged the other way (on the whole four-year record at once, the single-pool test the scorecard actually uses) that same stress spike does NOT hold; it's a not-found on the scorecard. The signal that held up across the whole record is a different one: the autonomic-variability reading (sleep-stress stdev), which cleared the project's own pre-registered discrimination bar even though it sits below the stricter cluster-corrected line here. Two honest cuts of the same data, and they agree on the thing that matters: none of these is a predictor.
The findings are still real; "many converging channels" was the overstatement. So we changed the headline rather than the evidence, which is the whole point of checking.
The same fact, read the other way
Collapsing seven readings to one factor is a discount when you're counting tests. But for a different question (is the watch producing real signal at all?) the same fact points the other way. If the wrist were spilling noise, there'd be no reason for seven readings to fold down onto a single autonomic factor that moves coherently, in the right direction, and in proportion to how bad the day was. Noise doesn't organise itself that neatly. So the collapse doesn't make the signal weaker; it tells you what it is: one real autonomic signal, measured several ways, not seven, but not zero.
What this page can't settle on its own is whether that one factor tracks anything outside the watch. That rested on an external anchor, the felt-state score, and the one independently-known autonomic event in the record. That event has now been tested: during a 14-day COVID infection the factor moved significantly, a qualified pass, and tellingly, one carried by these same correlated stress and body-battery legs. So 'one factor, several ways' isn't only internal book-keeping; it's what moved for a real event. See the COVID check.
The correlation values shown are the real ones the research has reported; the full signal-by-signal matrix is still to be exported. Statuses and the exact effective-N may sharpen, but the collapse is the finding.
The full cross-channel correlation work, in the research repo: cross-channel-correlation.md ↗