Every score, explained.

Recovery

Inputs. Overnight heart rate variability (rMSSD), resting heart rate and, where the device provides it, respiratory rate. Each is compared with a rolling baseline of your own recent nights, so a value is only low if it is low for you.

Ceilings. The physiology score is then capped by three ceilings: last night's sleep against your need, your recent training load, and anything in your journal that is known to depress recovery. The final score is the lowest of the four. This is deliberate. A great HRV on five hours of sleep does not make a 90% morning, and a bad night should not be counted twice as both low HRV and low sleep.

What it needs. A device worn to bed. Around a week of nights to build a baseline, or existing Apple Health history to seed one.

What it cannot do. Tell you why. HRV falls with illness, alcohol, heat, stress and hard training alike. The journal is how you find out which.

HRV

Sixteen uses rMSSD, the root mean square of successive differences between adjacent heartbeats, the standard short term measure of parasympathetic activity described in the 1996 Task Force guidelines and the 2017 Shaffer and Ginsberg review.

From a WHOOP strap, Sixteen receives beat to beat intervals over Bluetooth. Only adjacent beats are paired, artefacts are rejected with a standard percentage filter, and the night is pooled into one value. Pairing beats across gaps inflates HRV several fold, which is why a strict adjacency rule matters. From Apple Health, Sixteen reads the HRV summaries Apple Watch writes and builds the baseline from nightly averages.

The normal band is a rolling mean with a smallest worthwhile change, following the approach Plews and colleagues describe for monitoring athletes, so day to day noise is shown as noise.

Sleep

Four parts. Hours against need: need is your baseline duration plus recent debt plus an allowance for today's strain, following the general shape of the sleep need model that wearables use. Consistency: how much bedtime and wake time drifted from your usual. Efficiency: time asleep over time in bed. Sleep stress: overnight heart rate and HRV relative to your own overnight norm.

Sleep stages are taken from the device that recorded them. Sixteen does not estimate stages of its own. Naps are detected and counted. A night that is split is stitched back together, and a settled night is never overwritten by a provisional one.

Strain

Day strain is time in each heart rate zone, weighted by zone, in the family of Edwards' training impulse. Minutes below a floor do not count. Logged strength sessions add a muscular load term from session RPE and volume, so lifting is not scored as rest. The scale runs 0 to 21 and the target for the day is set from recovery. We do not calibrate to any other brand's number and do not claim equivalence.

Nutrition

Calories and protein logged against a target derived from your goal weight. Photo logging is an estimate you correct before saving. Diet quality is scored in the spirit of the Alternate Healthy Eating Index, which rewards whole foods and penalises sugar and processed meat, without any claim of clinical outcome.

Screen time

Sixteen uses Apple's Screen Time framework to shield the apps you choose. The screentime score summarises total time, first app opened and last app before bed. Earned minutes come from completed tasks and activity, and the bank never goes negative by design. Bedtime locks follow your sleep schedule and release in the morning.

Privacy and data

Scores are computed on your iPhone. Health data is read from Apple Health with your permission and is not sent to Sixteen's servers unless you turn on backup. WHOOP strap data goes from the strap to your phone over Bluetooth and nowhere else. You can delete everything from Settings.

Review

Last updated 2026-09-15. Corrections and questions: keshav@sixteen.life.

Sources

1. Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology. Heart rate variability: standards of measurement, physiological interpretation, and clinical use. Circulation, 1996.
2. Shaffer F, Ginsberg JP. An overview of heart rate variability metrics and norms. Frontiers in Public Health, 2017.
3. Plews DJ, Laursen PB, Stanley J, Kilding AE, Buchheit M. Training adaptation and heart rate variability in elite endurance athletes: opening the door to effective monitoring. Sports Medicine, 2013.
4. Edwards S. The Heart Rate Monitor Book. Polar Electro Oy, 1993.
5. Foster C et al. A new approach to monitoring exercise training. Journal of Strength and Conditioning Research, 2001. (Session RPE.)
6. Chiuve SE et al. Alternative dietary indices both strongly predict risk of chronic disease. Journal of Nutrition, 2012. (AHEI.)
7. Hirshkowitz M et al. National Sleep Foundation's sleep time duration recommendations. Sleep Health, 2015.