Svanu
Read your numbers.
What HRV, resting heart rate, recovery, sleep and caffeine numbers mean, with the ranges, the studies behind them, and how each score is computed.
50 articles · 8 topics · 0 of 50 medically reviewed
Heart rate variability
GuideWhat is a good HRV? Ranges and what your number means
What is a good HRV? Short resting RMSSD in healthy adults averages about 42 ms, but your own baseline matters more. See ranges and what moves your number.
HRV chart by age and sex
Heart rate variability chart by age for men and women. Average 5-minute RMSSD falls from about 40 ms at 25 to 34 to about 19 ms at 65 to 74. See the full table.
How to increase HRV: evidence-based levers
How to increase HRV with levers backed by studies: aerobic training, enough sleep, less alcohol, slow breathing and planned rest. What helps and how much.
Low HRV: what it means and common causes
Low HRV meaning explained: a reading below your own baseline often follows alcohol, poor sleep, hard training or illness. Causes and when to see a doctor.
Measure HRV with your iPhone camera, no watch needed
How to measure HRV with no watch: a finger on the iPhone camera records RMSSD in under a minute. How it works, how accurate it is, and how to get clean data.
Alcohol and HRV: how one drink shows in your recovery
HRV and alcohol: in 4,098 adults, overnight RMSSD fell 2 ms after about one drink and 13 ms after about seven. Why it happens, how long it lasts, what to do.
HRV during sleep: what is normal overnight
What is HRV sleep data telling you? HRV rises in deep non-REM sleep and falls toward waking levels in REM. Normal overnight patterns and how to read it.
RMSSD vs SDNN: which HRV number your app shows
RMSSD is the root mean square of successive differences between heartbeats, the main vagal HRV metric. How it differs from SDNN, with a worked example.
Good HRV for women and how the cycle changes it
Whats a good HRV for women? Average resting RMSSD is about 43 ms at 25 to 34 and 21 ms at 55 to 64. How age and the menstrual cycle shift your number.
Apple Watch HRV: how it is measured and how accurate it is
Heart rate variability Apple Watch data is stored as SDNN from readings taken while you are still. How to check it, what is new in 2026, how accurate it is.
Heart rate
GuideResting heart rate by age: chart for men and women
Resting heart rate by age for men and women, from US national survey data. See the average, the normal range, wearable values and when a number needs a doctor.
How to lower your resting heart rate
How to lower resting heart rate with evidence: endurance training, yoga, sleep and less alcohol. See how many bpm to expect and when a low rate needs a doctor.
Why is my resting heart rate going up?
Why is my resting heart rate going up? Check the common causes: illness, alcohol, short sleep, cycle phase, pregnancy and medicines, and when to see a doctor.
Sleeping heart rate: what is normal and what the dip means
Sleeping heart rate normally drops 10% or more below your awake rate. See typical values, the overnight curve, what raises it and when to see a doctor.
Heart rate when sick: how wearables spot illness early
Elevated heart rate when sick is common: fever adds about 8.5 bpm per 1 °C. See how heart rate, HRV and temperature change and when to see a doctor.
Check your heart rate on iPhone without an Apple Watch
How to check heart rate on iPhone without an Apple Watch: camera apps, AirPods Pro 3, manual pulse and other sensors, with accuracy for each method.
Heart rate recovery: what is a good number
Heart rate recovery is how fast your pulse drops after exercise. A drop above 12 bpm at 1 minute is the usual cut-off. See ranges and how to measure it.
Why am I getting sick so often? What your vitals can show
Why am I getting sick so often? Adults average 2 to 3 colds a year. See how sleep, stress and exercise relate to colds and when to see a doctor.
Recovery and training
GuideRecovery score explained: HRV, resting heart rate and your 7-day baseline
What a recovery score measures, how HRV, resting heart rate and sleep feed it, why your 7-day baseline matters, and how Whoop, Oura, Garmin and Apple compare.
How Whoop calculates recovery, and how to get a recovery score without Whoop
How the Whoop recovery score uses HRV, resting heart rate, sleep and respiratory rate, what the colors mean, and how to get a similar score without the band.
Apple Watch recovery score: how to get one on any model
Apple Watch recovery score explained: what the Readiness app needs, which models support it, and how older watches get a score from Apple Health.
What is strain? Daily strain score vs your target, explained
What does strain mean in fitness tracking? How a strain score measures daily load from heart rate and activity, how Whoop strain works, and how to set a target.
Acute:chronic workload ratio (ACWR) explained, with the evidence for and against
The ACWR compares your last 7 days of training load with your 3 to 6 week average. How to calculate it, the 0.8 to 1.3 range, and why researchers dispute it.
Overtraining symptoms: the signs your data can show first
Overtraining symptoms include lasting fatigue, falling performance, poor sleep and low mood. Learn the stages, what HRV can show and when to see a doctor.
Zone 2 heart rate: how to find yours, with a chart by age
Zone 2 heart rate is usually 60 to 70% of maximum heart rate. Find your zone 2 range by age, check it with the talk test, and see what the research says.
VO2 max by age: chart for men and women, and what a good score is
VO2 max by age chart from lab-measured US data. See percentiles for men and women, what counts as a good VO2 max, and how fitness changes with age.
Is Apple Watch VO2 max accurate? What the validation studies found
Apple Watch VO2 max accuracy: independent studies found it underestimates lab values by about 4.5 to 6 mL/kg/min. Why, and how to get a better estimate.
How to improve VO2 max: what the training studies show
How to improve VO2 max: intervals such as 4x4 minutes at 90 to 95% of max heart rate plus steady aerobic work raised VO2 max in studies. Plans and timelines.
Biological age: estimate yours from heart and fitness data
A biological age calculator compares heart and fitness data with your age. See which signals research links to aging, how estimates work and their limits.
Sleep
GuideSleep score: what it measures and what a good score is
A sleep score rates last night from 0 to 100 using duration, timing, wake-ups and stages. See what a good sleep score is on Apple, Fitbit, Oura and Garmin.
Apple Watch sleep score: how it works and how to read it
The Apple Watch sleep score rates each night 0 to 100 from duration, bedtime consistency and interruptions. Learn how points are earned and how to raise it.
What is core sleep on Apple Watch?
What is core sleep? On Apple Watch, core sleep is light sleep (stages N1 and N2), often over half the night. Normal amounts and how it compares to deep sleep.
How to get more deep sleep: what the evidence supports
How to get more deep sleep: protect the first half of the night, cut late caffeine, skip evening alcohol, exercise, keep cool. What studies show.
How much deep sleep do you need? Normal amounts by age
How much deep sleep do you need? Most adults get about 10 to 25 percent of sleep as deep sleep, roughly 42 to 105 minutes in 7 hours. Ranges by age and caveats.
How much REM sleep should you get?
How much REM sleep should you get? Adults typically spend about 15 to 25 percent of sleep in REM, around 1 to 1.75 hours in a 7-hour night. Ranges and tips.
Sleep debt: how to calculate it and pay it back
Sleep debt is the gap between the sleep you need and the sleep you get. Learn to calculate it over 14 nights, why weekends fall short, and how to repay it.
Sleep cycle alarm: does waking in light sleep work?
A smart alarm wakes you inside a time window, aiming for lighter sleep to cut grogginess. See how sleep cycle alarms work and what research says about them.
How to track sleep on Apple Watch, and what each metric means
How to track sleep on Apple Watch: set up Sleep in Health, turn on Track Sleep, charge to 30 percent and wear it to bed. Plus what each metric means.
Stress score: how wearables measure stress from HRV
A stress score estimates body stress from heart rate and HRV, often on a 0 to 100 scale. How Garmin, Oura and Apple Watch handle it, and what it misses.
Caffeine and nutrition
GuideHow long does caffeine stay in your system? Half-life, timeline and bedtime math
How long does caffeine stay in your system? Half-life is about 5 hours, so a quarter of a dose is still there after 10. See the timeline and bedtime math.
When to stop caffeine before bed: the evidence-based cutoff time
When to stop drinking caffeine before bed: a 2023 meta-analysis says at least 8.8 hours for a regular coffee and 13.2 hours for a strong pre-workout dose.
Caffeine in coffee, espresso, matcha and tea: a sourced chart
How much caffeine in a cup of coffee? About 95 mg per 8 oz brewed cup, 63 mg per espresso shot. Sourced chart for coffee, matcha, green tea, black tea, decaf.
AI photo calorie counting: how accurate is it?
Is a photo calorie counter accurate? In a 2025 study, AI estimates from photos alone missed by about 31% on average, and by 14% when ingredients were added.
Menstrual cycle
GuideLuteal phase: symptoms, temperature, HRV and heart rate
Luteal phase symptoms include mood changes, bloating and breast tenderness. See how temperature, resting heart rate and HRV shift before a period.
Apple Watch and Apple Health
GuideBest health apps for Apple Watch (October 2026)
The best health apps for Apple Watch as of October 2026, compared by price, recovery, sleep and privacy: built-in apps, Athlytic, Bevel, AutoSleep and more.
Comparisons
GuideBest Whoop alternatives with no subscription (October 2026)
The best Whoop alternative depends on what you wear. Compare no-subscription bands, rings and Apple Watch by price, recovery data and 2-year cost.
Best Oura Ring alternatives, including no ring at all (October 2026)
Looking for an Oura ring alternative? Compare rings without a subscription, bands, Apple Watch and a phone-only option by price and 2-year cost.
Svanu vs Bevel vs Athlytic (and how they compare with Whoop)
Bevel vs Whoop vs Athlytic as of October 2026: price, hardware, recovery and strain scores, nutrition, AI coach and privacy, from each official page.