The evidence
Clarity starts
with the evidence.
The fundamentals deserve close attention. So do the limits of what we know.
These sources explain the areas we focus on. They do not measure Zensu’s results or predict your outcome.
Editorial review
01 Observational evidence
Fitness is worth measuring.
A 2009 meta-analysis associated each 1-MET higher level of cardiorespiratory fitness with 13% lower relative risk of death from any cause. A 2024 overview of meta-analyses, covering 199 cohort studies and more than 20.9 million observations, found each 1-MET higher level associated with 11–17% lower all-cause mortality, and high versus low fitness with roughly half the risk (hazard ratio 0.47).
The limit: These findings concern fitness levels in populations. They do not promise a particular benefit from improving one person’s score. The 2024 overview rated the certainty of its evidence from very low to moderate.
02 Observational evidence
Strength belongs in the plan.
A 2022 analysis of seven cohort studies (263,058 adults) associated muscle-strengthening activity with 15% lower relative all-cause mortality risk compared with none, independent of aerobic activity. The largest association appeared at around 30–60 minutes a week.
The limit: The authors rated certainty for this outcome very low. Observed associations around weekly training time do not establish an ideal dose for everyone.
Read: Momma et al., BJSM, 2022
03 Cohorts & population modelling
Daily habits matter over decades.
A 2018 US study linked healthier lifestyle patterns with longer modelled life expectancy. Its frequently cited 12–14-year difference compares groups at age 50.
The limit: It does not predict extra years gained by an individual. The historical score included moderate alcohol intake; this is not a reason to start drinking. WHO identifies cancer risk even at low consumption.
Read: Li et al., Circulation, 2018 · WHO: alcohol and cancer
04 Expert consensus
Light has timing as well as brightness.
For healthy adults on regular daytime schedules, a 2022 consensus recommends these reference targets, measured at the eye:
- During the day
- ≥250 lux melanopic EDI
- Three hours before bed
- ≤10 lux melanopic EDI
- Sleep environment
- ≤1 lux melanopic EDI
Melanopic equivalent daylight illuminance accounts for the light’s spectrum. It is not interchangeable with an ordinary lux reading.
The limit: These are not diagnostic thresholds, night-shift guidance or a jet-lag treatment protocol. Visual safety and accessibility requirements take precedence; lighting must allow safe movement.
05 Clinical travel guidance
The itinerary changes the timing.
CDC guidance describes planned sleep timing and appropriately timed light as tools for adapting to a destination. Direction, time zones, trip length and usual sleep schedule matter.
A 2025 analysis of about 1.5 million nights of wearable data, from 64,847 trips, found sleep duration back within about 12 minutes of normal after roughly two days, while sleep timing still had not fully returned to normal 15 days after travel. Eastward trips and more time zones meant more disruption.
The limit: One light schedule does not suit every journey. Medication and melatonin decisions need individual consideration. The wearable study describes ring users, probably mostly on holiday. It was not a study of flight crew; atypical baseline sleep episodes were excluded. A travel plan is not a fitness-to-fly assessment.
Read: CDC Yellow Book 2026: jet lag · Willoughby et al., SLEEP, 2025 · Our guide to the study
06 Clinical evidence & screening principles
Every test needs a reason.
A clinician weighs screening benefits and harms against your age, history and risk, including what happens after a result.
The limit: Longer survival measured from diagnosis can simply reflect earlier detection. Screening can also find disease that would never cause harm. Stage-specific survival alone does not prove that a screening test saves lives.
07 Emerging research
A score needs context.
Biological-age tests remain an evolving field. Methods can produce different estimates, and researchers continue to examine their reliability and clinical usefulness.
The limit: A lower score does not by itself establish better health or a longer life. Results need interpretation alongside clinical history and established health measures.
Read: Nature Medicine, 2024: validation · Nature Aging, 2024: clinical translation
08 Surveys, cohorts & expert review
Flying for a living leaves a signature.
In an anonymous survey of 3,765 US pilots, 56% reported avoiding healthcare at some point for fear of losing their medical certificate.
Pooled studies found about twice the melanoma incidence among pilots and cabin crew, compared with the general population. In a survey of 328 GCC airline pilots, 68% scored in the severe-fatigue range and 29% screened at high risk of obstructive sleep apnoea.
In a 2026 report, the US National Academies found that flight crews receive among the highest occupational doses of ionising radiation in the United States, though estimated annual doses generally remain below recommended limits. Because the risk is cumulative, the report recommends tracking dose across a career.
The limit: The surveys are self-selected and self-reported. Most melanoma data were collected between the 1970s and 1990s, and the studies cannot separate altitude from sun exposure and lifestyle. None of these findings predicts an individual pilot’s health, and none bears on fitness to fly, which sits with the AME and the regulator.
Read: Hoffman et al., JOEM, 2022 · Sanlorenzo et al., JAMA Dermatology, 2015 · Miura et al., British Journal of Dermatology, 2019 · Aljurf et al., Sleep & Breathing, 2018 · National Academies, 2026