Nocturnal Heart Rate as a Digital Biomarker

Nocturnal heart rate is the heart rate recorded while a person is asleep, usually reported as an overnight average, an overnight minimum, or the shape of the curve across the night. It is distinguished from resting heart rate by the conditions rather than the physiology: during sleep the person is lying down, not exerting, not caffeinated, not talking and not attending an appointment.

That near standardisation is the entire argument for the measure. Most of the variance in a daytime heart rate reading comes from behaviour rather than from cardiac status, and sleep removes most of that behaviour. Night to night comparison within one person is therefore substantially cleaner than day to day comparison, which is why deterioration signals often show up in nocturnal values before daytime ones.

Consumer wearables estimate it optically during periods the device classifies as sleep, which means the measure inherits any error in the sleep detection itself. If the algorithm misclassifies quiet wakefulness as sleep, waking heart rate is folded into the nocturnal average. Devices differ in whether they report a mean across the whole sleep period, a minimum, or a value from a specific stage, and those choices produce different numbers from the same night.

Validation work against electrocardiography during sleep has shown reasonable agreement for wrist optical sensing under these conditions, better than during activity, which is consistent with motion being the main source of optical error. As with resting heart rate, the device and its firmware version should be recorded, and values should not be pooled across device types.

The strongest use is trend monitoring. A sustained rise in nocturnal heart rate is an early physiological signal in infection, in heart failure decompensation and in overtraining, and because it is measured under repeatable conditions the rise is easier to distinguish from noise than a daytime change would be. Elevated night time heart rate has been associated with cardiovascular mortality in monitored cardiac populations.

It also sits at the intersection of sleep and cardiology, which is why it appears in both. In sleep research it is a physiological correlate of sleep quality and arousal burden; in cardiology it is a rate control and prognostic measure recorded without the confounding of daytime activity. It is reported alongside instruments such as the Pittsburgh Sleep Quality Index or the NYHA functional class depending on which question the study is asking.

The evidence base is thinner than for resting heart rate. Nocturnal heart rate appears in relatively few registered trials, its prognostic literature comes largely from monitored cardiac populations rather than from wearable cohorts, and there is no established threshold for what size of change matters.

The measure inherits sleep detection error, and it is confounded by everything that affects sleep itself, including alcohol, late meals, room temperature, illness and shift work. It is also not comparable between devices, since each defines the nocturnal value differently. Treat it as a sensitive within person trend signal rather than as an absolute measurement with an established clinical meaning.

• Jiang J, et al. Association between nighttime heart rate and cardiovascular mortality in patients with implantable cardioverter-defibrillator. Heart Rhythm. 2023. pubmed.ncbi.nlm.nih.gov
• Haghayegh S, et al. Accuracy of PurePulse photoplethysmography technology of Fitbit Charge 2 for assessment of heart rate during sleep. Chronobiol Int. 2019. pubmed.ncbi.nlm.nih.gov
• Quer G, et al. Inter- and intraindividual variability in daily resting heart rate and its associations with age, sex, sleep, BMI, and time of year. PLoS One. 2020. pubmed.ncbi.nlm.nih.gov
• Nanchen D. Resting heart rate: what is normal? Heart. 2018. pubmed.ncbi.nlm.nih.gov