Heart rate variability (HRV)
The tiny changes in time between your heartbeats. Apple Watch records it as SDNN, in milliseconds, from short readings when you're still.
A healthy heart isn't a metronome. The gap between beats stretches and shrinks all the time, and HRV measures how much. Apple describes HRV as a possible indicator of stress and recovery.
Apple Health stores HRV as SDNN: the spread of the intervals between beats, in milliseconds. Apple Watch takes these readings on its own at moments when you're still, so how many you get depends on how active your day is.
Your number only makes sense next to your own. SDNN grows with longer recordings and depends on age, sex and fitness, so comparing it with a friend's, or with another device, tells you very little. That's why there's no typical range here.
It tends to dip for a day or two after a hard workout, and it drops during sleep after drinking alcohol, even a little. It drifts lower with age. It tends to rise as your aerobic fitness improves.
Single readings bounce around a lot, so Pajdo works with a 7-day average and compares it with your usual range from up to the last 60 days.
What tends to push it up
- Better aerobic fitness
What tends to pull it down
- Hard training in the last day or two
- Alcohol, even a small amount
- Getting older
Worth a try
Habits that support HRV
A meta-analysis of trials in healthy adults found that exercise training, kept up for weeks to months, raised HRV compared with not training. Walking, running, cycling and yoga all counted.
Alcohol pulls the other way. In a study of over 4,000 people, even a small amount was linked to lower HRV in the first hours of sleep. Some studies suggest losing sleep nudges it down too, though results are mixed.
HRV varies with age, sex and fitness, so compare it with your own usual.
Try it: pick a workout you like, do it a few times a week for a couple of months, and watch your HRV trend in Pajdo.
Keep reading
Sources
- HKQuantityTypeIdentifier.heartRateVariabilitySDNN, Apple Developer Documentation
- Monitor your heart rate with Apple Watch, Apple Support
- An overview of heart rate variability metrics and norms (Shaffer and Ginsberg 2017), Frontiers in Public Health
- Cardiac parasympathetic reactivation following exercise: implications for training prescription (Stanley et al., 2013), Sports Medicine
- Acute effect of alcohol intake on cardiovascular autonomic regulation during the first hours of sleep: observational study (Pietilä et al. 2018, 4,098 employees), JMIR Mental Health
- Effects of exercise training on heart rate variability in healthy adults: a systematic review and meta-analysis of randomized controlled trials (Amekran and El hangouche 2024), Cureus
- Effects of sleep deprivation on heart rate variability: a systematic review and meta-analysis (Zhang et al. 2025), Frontiers in Neurology
Checked against these sources on October 11, 2026.
This is general wellness information, not medical advice. If a number worries you, or stays away from your usual for a while, talk to a doctor.