How Cortisol+ works: HRV and sleep-based cortisol tracking

The short version

Cortisol+ does not measure cortisol directly. No consumer wearable can — cortisol is a hormone measured in blood, saliva, or urine. Instead, Cortisol+ estimates your cortisol pattern from validated biometric correlates collected by Apple Watch.

The signals we use

Heart rate variability (HRV / SDNN)

HRV is the variation between successive heartbeats. It's the most direct wearable proxy for autonomic stress and shows strong inverse correlation with cortisol in peer-reviewed research. Apple Watch records SDNN (standard deviation of normal-to-normal intervals) — the HRV metric with the largest evidence base for stress assessment.

Sleep architecture

REM and deep sleep deficits raise next-morning cortisol. The relationship is bidirectional: high cortisol fragments sleep, and fragmented sleep raises cortisol the next day. Cortisol+ tracks sleep stage transitions from Apple Watch's sleep classification model and surfaces the correlation explicitly.

Resting heart rate trend

Chronic elevation in resting heart rate (5+ bpm above your 30-day average for 5+ days) is one of the cleanest signals of chronic stress load and is a leading indicator of HPA axis strain.

Activity and recovery balance

Cumulative training load without recovery raises chronic cortisol. We track the balance between strain and recovery using activity ring data and HRV recovery patterns.

Wrist temperature

On Apple Watch Series 8+, wrist temperature deviation surfaces circadian disruption — a strong indicator of cortisol-rhythm disturbance, particularly in shift workers and perimenopause.

Blood oxygen

Secondary signal — drops in nocturnal SpO₂ correlate with sleep apnea and fragmented sleep, both of which drive cortisol up.

How the cortisol score is calculated

Cortisol+ combines these signals into a 0–100 cortisol score, updated continuously throughout the day, and mapped to four bands (Low, Normal, Elevated, High). The score reflects your pattern — how your current biometric state compares to your personal baseline established over your first 7 days of wearing the watch.

This means your score is personalized to YOU. A 45 on your account doesn't mean the same as 45 on another account — it means something specific about how you are right now compared to how you usually are.

How accurate is the estimation?

Honest assessment: Cortisol+ is accurate for trend tracking — catching changes in your cortisol pattern over days and weeks. It is NOT accurate for absolute cortisol values in ng/mL or mcg/dL. The underlying HRV-cortisol correlation in the literature is strong enough to detect meaningful changes (elevated/depressed states) but not precise enough to replace a lab test.

For absolute values, get a one-time 4-point salivary cortisol test (see our testing guide). For continuous pattern tracking and response to interventions, Cortisol+ is the right tool.

What we explicitly don't do

• We don't diagnose Cushing's syndrome, Addison's disease, or any endocrine disorder
• We don't replace a saliva, blood, or urine cortisol test
• We don't make medical claims about your cortisol level — only about your pattern
• We don't sell your health data; everything runs on-device

The right use case

Use Cortisol+ to:

• See how your sleep last night affected your cortisol today
• Identify what raises and lowers your specific cortisol pattern
• Measure whether a supplement, lifestyle change, or intervention is actually moving your trend
• Catch overtraining or burnout trajectory weeks before subjective symptoms peak
• Build a baseline before a saliva test so the results are interpretable

Use a doctor and clinical testing for:

• Diagnosing suspected adrenal disease
• Absolute cortisol values
• Any persistent symptom that worries you