In one sentence
How fast your heart beats while you are asleep, which is the closest thing to your heart at rest that can be measured every day.
Why it matters
The resting heart rate is set by the heart's own pacemaker and by the nervous system's brake on it, so it reflects fitness, recovery and strain at once. In a meta-analysis of 46 cohort studies with 1,246,203 adults, each 10 beats per minute of higher resting heart rate was associated with 9 percent higher all-cause mortality, and people resting above 80 beats had 45 percent higher mortality than the lowest group (Zhang 2016). In trained people a lower rate at rest is one of the clearest marks of an efficient heart. The overnight figure matters more than the daytime one because it is taken under the same conditions every night, lying still in the dark, which is the only way a daily series can be compared with itself.
What moves it
Downward over months: aerobic fitness. Upward on a given night, and in roughly this order of size: a fever, by about 8.5 beats for each degree Celsius in 27 young men recorded through an acute febrile infection, with the rate staying high even during sleep (Karjalainen 1986); an evening of heavy drinking, by about 6 percent in 9 million free-living readings (Altini 2021); a day of being unwell, also about 6 percent in the same data; a large or late meal, a hot room, dehydration, a hard session that day, a short night before, and a stressful day. It also sits about 1.6 percent higher in the second half of the menstrual cycle than in the first (Altini 2021; Alzueta 2022), and higher at altitude.
How to measure it well
Wear the same ring or band every night; the overnight figure is only comparable with itself on the same device. Nothing else is needed, except to log the things that move it (alcohol, a late meal, training, your period) so VT can take them into account.
How Vitality Tracker uses it
It is one of the four main overnight readings Symptom Radar compares with your own baseline from the last 30 nights each morning, with sick days and the three after them left out of the baseline, and with your training (when you have set a training goal), alcohol, a late meal, a late bedtime and your period taken into account before anything is said. It is also an input to the recovery side of Capacity. A change of device starts a new series; VT never compares a new device's nights with an old one's, and only nights from your usual sleep device count toward the baseline. Once Symptom Radar is running, a night below 40 or above 100 beats per minute adds a safety note to the morning's result, whatever your baseline.
How far to trust the number
Measured. A finger ring read the nightly average within 0.63 beats of an electrocardiogram in 49 adults (Kinnunen 2020), and wrist sensors at rest err by under 5 percent (Shcherbina 2017). Night-to-night noise is a few beats, so read the seven-night average and treat a single night's jump as a question, not an answer; the answer is usually in what you logged the evening before.
Sources
- Zhang D, Shen X, Qi X. Resting heart rate and all-cause and cardiovascular mortality in the general population: a meta-analysis. CMAJ 2016;188:E53–63. doi:10.1503/cmaj.150535
- Karjalainen J, Viitasalo M. Fever and cardiac rhythm. Arch Intern Med 1986;146:1169–71 (PMID 2424378)
- Altini M, Plews D. What is behind changes in resting heart rate and heart rate variability? A large-scale analysis of longitudinal measurements acquired in free-living. Sensors 2021;21:7932. doi:10.3390/s21237932
- Alzueta E, et al. Tracking sleep, temperature, heart rate, and daily symptoms across the menstrual cycle with the Oura ring in healthy women. Int J Womens Health 2022;14:491–503. doi:10.2147/IJWH.S341917
- Kinnunen H, Rantanen A, Kenttä T, Koskimäki H. Feasible assessment of recovery and cardiovascular health: accuracy of nocturnal HR and HRV assessed via ring PPG in comparison to medical grade ECG. Physiol Meas 2020;41:04NT01. doi:10.1088/1361-6579/ab840a
- Shcherbina A, et al. Accuracy in wrist-worn, sensor-based measurements of heart rate and energy expenditure in a diverse cohort. J Pers Med 2017;7:3. doi:10.3390/jpm7020003
- Vitality Tracker, How it works: Symptom Radar.