What does device-free measurement rely on?
The strongest predictors of ageing rate are behavioural: sleep regularity, daily movement, stress load, alcohol and tobacco use, subjective energy. All five can be captured in a 30-second daily log.
Your phone already carries a motion sensor. Daily step totals from a pocketed phone track closely with wrist devices — without extra hardware.
What are the advantages?
Consistency: nothing to charge, wear or lose. The biggest threat to long-term data quality is abandoning the device after a few weeks.
Cost: the same behavioural curves without hardware or subscriptions.
Privacy: summary entries you choose to log instead of a continuous heart-rate and location stream.
Where is it weaker, and how do you compensate?
Device-free tracking has no sleep staging, heart-rate variability or continuous pulse. Subjective sleep quality plus a manual morning resting pulse recovers most of the useful signal.
Count your morning pulse for 60 seconds once a week, and run a sit-to-stand and single-leg balance test monthly. Those two habits replace most of the practical value of a wearable, for free.
Who is it for?
People who dislike wearing devices, sleep badly with something on the wrist, do not want another subscription, or simply prefer fewer numbers.
fjt was built for exactly that: nothing on your wrist, just phone step data and a short daily check-in.
