Why it matters
If you’re fatigued, an easy day or rest day fixes it. If you’re getting sick, training makes it worse — even easy training. Getting this wrong costs days or weeks. Your device shows the same yellow score either way.
The fatigue pattern
HRV suppressed after hard training block. RHR slightly elevated (2–4 bpm). Sleep quality okay. No temperature change. Feel: tired legs, low motivation, but no body aches. Recovery timeline: 1–3 days of rest.
The illness pattern
HRV declining WITHOUT training load explanation. RHR elevated 48+ hours (even on rest days). Temperature deviation positive (even 0.3–0.5°C). Respiratory rate elevated. Feel: general malaise, not localized to muscles.
HRV managed to indicate sicknesses before I actually feel them. HRV will drop, I’ll feel fine… Third day I’m dying.
How Rudder detects the difference
Illness suspicion composite: temperature RED + RHR 48h elevation = “likely.” Breathing rate elevation + RHR amber = “possible.” HRV declining without training load change = early warning.
The key: fatigue has a training load explanation. Illness doesn’t.
What to do
Fatigue: easy day, extra sleep, resume in 1–2 days.
Illness suspicion: full rest, hydrate, no training until symptoms resolve.
When in doubt: the warmup gate. If you feel better after 15 minutes of easy movement, it’s fatigue. If you feel worse, stop.
The science
HRV suppression without load change predicted illness 1–3 days before symptoms (Hamlin 2019). RHR elevation for 48+ hours is a stronger signal than a single morning reading. Temperature deviation is the most specific illness marker from consumer wearables.
Related
Frequently asked
- How do I know if I'm sick or just tired?
- Normal training fatigue improves with a warmup and clears within 24-48 hours. Illness fatigue persists or worsens, often accompanied by a 3+ day HRV decline without a training load explanation. Rudder tracks these patterns and flags illness suspicion early.
- Can Rudder detect illness before I feel symptoms?
- Sometimes. When HRV declines for 3+ days without matching training load, combined with elevated resting heart rate or temperature deviation, Rudder flags potential illness. It caught this pattern 2 days before symptoms in our testing.
- Should I train through a cold?
- Depends on severity. Mild symptoms above the neck with stable HRV: light activity may be fine. Below-the-neck symptoms, fever, or declining HRV: back off. Rudder makes this call based on your specific data, not a generic rule.