Definition
Biological age expresses the functional state of your body in years. Chronological age measures the calendar; biological age summarises how well your cells, vessels, muscles and brain work.
Two people the same age can be 15-20 years apart. Most of that gap is habits, not genetics.
Measurement: with and without a lab
Lab methods: epigenetic clocks (DNA methylation) and blood-panel formulas such as PhenoAge. Most accurate, but costly and realistic once or twice a year.
Lab-free methods: behavioural data (sleep regularity, steps, stress, subjective energy) plus functional tests (resting pulse, sit-to-stand, single-leg balance). Less precise, but measurable daily — which is what actually changes behaviour.
No device required
You do not need anything on your wrist. Your phone's motion sensor already records daily steps, and the remaining signals fit into a 30-second daily log.
The device-free advantage is continuity: no hardware to charge, wear or replace, so the data series does not break.
The four biggest levers
Sleep regularity: a fixed wake time outweighs total duration.
Daily movement: benefit rises steeply to about 7,000-8,000 steps, then flattens.
Muscle and strength: two resistance sessions a week buffer ageing best.
Stress and recovery: chronic stress raises inflammatory markers and doubles its impact by degrading sleep.
A 30-day plan
Week 1: fix your wake time and get 10 minutes of morning daylight.
Week 2: raise your step floor to 7,000 and walk 10-15 minutes after meals.
Week 3: add two 20-30 minute resistance sessions.
Week 4: set a caffeine cut-off, a screen cut-off and one full rest day.
Judge the result on the four-week average, not single days.