Does restarting training tend to precede illness?

Central question

• Does resuming regular workouts predict getting sick?
• Do other variables predict getting sick?

Planning considerations

• In the first upload of data, we have no indication of when he actually got sick.
  • His Apple Health export records no colds. Its only symptom entries are nine digestive ones, all in one week of June 2023 (streamed inventory of the whole export).
  • He sent no separate list of dates.
• So we will first try to detect illness episodes automatically.
  • We came up with an educated guess at automatic illness-detection criteria, based on the available data.
  • Available data, from his Apple Watch unless noted:
    • resting heart rate, one a day: 2,196 readings, 2020-09 → 2026-09
    • heart rate variability (HRV): 22,342 readings, 2020-09 → 2026-09
    • breathing rate during sleep: 85,803 readings, 2021-09 → 2026-09
    • wrist temperature, one a night (second watch only): 728 readings, 2024-09 → 2026-09
    • workouts: 1,149 records from several apps, 2020-10 → 2026-08
    • not used for detection: 319 ECG spot checks (2024-09 →); Withings weight and blood pressure (its sleep-mat data is not in the export); Ottai CGM (2026-03 →, too recent)
  • Automatic illness detection criteria, version 1:
    • a night is flagged when, compared with his own nights 3–30 before it, resting heart rate is at least 1 SD above normal, and HRV at least 1 SD below, and breathing rate or wrist temperature at least 1 SD above;
    • an illness episode is at least 2 flagged nights, joined across gaps of up to 2 nights; a new episode needs 7 clear nights after the last one.
  • We officially registered this definition before any illness signal had been computed, to keep ourselves honest.
    • Registered definition identifier: 2026-09-29 16:44 Pacific, git commit 9b2061c.
    • Otherwise we could guess "a million times" and pick the guess that fit the data best.
    • That would be bad because, with enough guesses, one of them will show a link by pure chance — flip enough coins and one of them lands heads ten times in a row. The test would then measure how hard we searched, not whether the gym matters. And a definition tuned to fit these five years would not recognise his next cold.
• Missed Tuesday/Friday trainer sessions cannot be the illness label.
  • A missed session is the gym stopping, so "training comes before illness" would be true by construction.
  • We keep them as a cross-check only.
  • Checking the data for that schedule: gym sessions starting 05:00–07:59 fall on Mon 62, Tue 64, Wed 40, Thu 61, Fri 50, Sat 4, Sun 0. Tuesday and Friday do not stand out, and most early sessions start in the 7 o'clock hour, not the 6. An early Tuesday/Friday pattern appears only in 2026 (19 of that year's 39 gym sessions). So the trainer schedule is not yet visible in the data; we need to ask him when it began.
• Hard training looks like illness in these signals.
  • A heavy week at the gym also raises resting heart rate and lowers HRV.
  • That is why version 1 requires a third signal, and at least two nights: recovery from a workout takes about one.
• Season is a rival explanation.
  • Colds peak in winter. If gym restarts also clustered in autumn, gym and colds would line up without either causing the other.
  • The test compares each restart with random dates in the same weeks of other years.
  • Result: restarts do not cluster in autumn or winter. By month they fall Jan 3, Feb 1, Mar 2, Apr 1, May 4, Jun 1, Jul 2, Sep 2, Oct 2, Nov 3, Dec 1. And the season-matched test gave almost the same answer as a test ignoring season (p = 0.33 against 0.31). Season changed nothing here, partly because there was no gym effect for it to explain.
  • Restarts by month and by season (northern hemisphere: winter is December–February). The dashed line is what an even spread across the calendar would give.

• "Resuming regular workouts" needs a working definition.
  • Ours, version 1: a restart is a gym session with no gym session in the 14 days before it. This is our choice; he gave no length of break.
  • Writing it down this way shows a gap: 3 of our 22 restarts were a single session with nothing after it (2023-05-02, 2024-04-29, 2026-01-04). One visit is not "resuming regular workouts".
  • We do not change version 1 now that results exist. A version 2 that requires, say, a second session within 14 days would be registered as a new question, with its own date.
• Definition of a gym session.
  • A gym session is inferred from the workout type, not from the location: Apple Health does not record location for indoor workouts.
  • The raw records: the export holds one workout record per workout, 1,149 in all. Each carries Apple's type string (for example HKWorkoutActivityTypeTraditionalStrengthTraining), the app that recorded it, and its start and end times, read as his local clock.
  • The type filter, fixed in the registration: a workout counts as gym if its type is traditional strength training (181 records), functional strength training (114 records), cross training (8 records), elliptical (136 records). Everything else is not gym — walking, tai chi, yoga, paddling, cycling, and Gentler Streak's untyped "Other".
  • Merging duplicates: the Apple Watch, Gentler Streak and Motra often each record the same visit. Records that overlap in time, or come within 5 minutes of each other, are merged into one session. That turns 439 records into 355 sessions, which fall on 256 different days inside the analysis window.
  • Where this can be wrong: type is not place. An elliptical session could be at home, and a gym visit logged under another type would be missed. The export cannot tell us which.
  • "Other" is the biggest unknown: Gentler Streak wrote 226 untyped workouts, and all of them are excluded. If many are gym visits, we are undercounting the gym. Not yet checked; if they are, the fix is a version 2 of this definition, registered separately.
• Only the first central question is confirmatory.
  • "Do other variables predict getting sick?" was not registered with a specific variable. Anything found there is a lead, to be confirmed on new data, not a finding.
  • Leads so far: none. No other variable has been screened yet. The only extra runs were three variations on the gym question itself, named in advance as exploratory, and none showed a link: without the third signal, rate ratio 1.16, p = 0.52; strength sessions only, rate ratio 1.83, p = 0.33; at-risk nights 7–14 only, rate ratio 0.00, p = 1.00.