Trends
A trend is a steady change over weeks, not one odd day. Pajdo only calls one when the change is clearly more than everyday noise.
Your numbers wobble from day to day. Resting heart rate, for example, typically moves a few bpm within a week for no special reason. One high day is just a day.
To find a trend, Pajdo fits the best straight line through your last 28 days and needs at least 14 days with data (VO2 max gets 90 days and needs 3 readings spanning at least 8 weeks). Every line comes with a confidence interval: the range the real change most likely sits in. Pajdo uses a 99% interval, stricter than the common 95%, because he checks several metrics every day and wants fewer false alarms.
A trend counts only when that whole interval sits on one side of zero and the change across the window is big enough to matter, like 3 bpm for resting heart rate or 30 minutes for sleep.
Fewer days or bigger wobbles make the interval wider, so a small real change can take a few weeks to show up.
Worth a try
Mix steady cardio with intervals
Apple says most people can improve VO2 max by increasing the intensity and frequency of cardio exercise. A meta-analysis in healthy adults aged 18 to 45 found both steady endurance training and interval training brought large gains, with intervals a little ahead.
Apple Watch estimates cardio fitness during Outdoor Walk, Outdoor Run and Hiking workouts, so those keep your readings coming.
Try it: keep your regular walks or runs, and once a week swap one for intervals, like a few minutes brisk then a few minutes easy. Give it weeks, not days.
Keep up regular endurance exercise
A meta-analysis of 191 studies in healthy people found that exercise lowered resting heart rate. Endurance training and yoga showed the clearest effect in both men and women.
It's a slow change, and plenty of things nudge daily readings around, so look at the trend rather than one morning.
Try it: pick an endurance activity you like, such as brisk walking, cycling or swimming, and do it on most days for a couple of months. Then check the resting heart rate trend from Apple Watch in Pajdo.
Keep reading
Sources
- Using the confidence interval confidently, Journal of Thoracic Disease (Hazra 2017)
- Inter- and intraindividual variability in daily resting heart rate and its associations with age, sex, sleep, BMI, and time of year, PLOS ONE (Quer et al. 2020)
- Track your cardio fitness levels, Apple
- Effectiveness of high-intensity interval training and continuous endurance training for VO2max improvements (Milanović, Sporiš and Weston 2015), Sports Medicine
- Effects of exercise on the resting heart rate: a systematic review and meta-analysis of interventional studies (Reimers, Knapp and Reimers 2018), Journal of Clinical Medicine
Checked against these sources on September 15, 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.