The immune response starts before the symptoms. While you still feel nothing, your body is already working differently — and at night that shows up best: you lie still, you drink no coffee, you are not stressed about work. Night is the only time of day when readings can be compared honestly.

Four night signals

  • Skin temperature. Rises by a few tenths of a degree — long before a thermometer shows a fever.
  • Resting heart rate. The heart starts working a little faster even in sleep.
  • Variability (HRV). Falls: the nervous system switches into fighting mode.
  • Blood oxygen. Dips if your breathing at night has got worse.

Compared with your baseline, not a table

None of these readings has a “correct” value for everyone. One person has a resting heart rate of 48, another 68 — and both are healthy. So we work out your personal corridor from past nights and look at whether today's reading has left it.

Why a coincidence is required

On its own, any single signal means nothing. Temperature rises in a hot bedroom and with your cycle. HRV falls after a glass of wine and after a hard workout. Oxygen dips if you slept on your back.

So the widget stays quiet while there is only one signal, and speaks only when two or more readings move towards illness in the same night. That coincidence is rare — and it usually happens a day before the first symptoms.

This is not a diagnosis

The widget does not tell a cold apart from overtraining, missed sleep or a hangover: the body reacts to all of them in a similar way. It says one thing — today you are not at your baseline. If things get worse, go to a doctor, not to a band.

What to do when the widget fires

  • Cancel the workout. Load right now will slow recovery, not “sweat it out”.
  • Go to bed earlier than usual: sleep works better than any supplement right now.
  • Drink more water during the day.
  • Look again tomorrow. If the signals are back to baseline, it was a one-off night.