Measure by Measure: Resting Heart Rate Across the 24 Hour Cycle
Abstract
Background: The development of photoplethysmography (PPG) sensors, typically found in wrist worn devices, offers the opportunity to continuously monitor heart rate in large populations in real world settings. Resting heart rate (RHR) is known to be an important biomarker of morbidities and mortality, but no universally accepted definition nor criteria for its measurement exist. In this study, we provide a working definition of resting heart rate and examine how heart rate at rest varies across the 24 hour cycle.
Methods: 433 healthy subjects wore a wrist device that measured activity and heart rate, day and night, for up to 3 months. Heart rate at rest (sedentary heart rate) was recorded, the duration of rest needed for RHR to stabilize was ascertained, and the true lowest RHRs for the sample population during the day and the night were established. The variation of RHR through the 24 hour cycle was also assessed. The sample was also subdivided according to their daily activity levels for subset analysis.
Results: Of the 26,758 days and nights available for measurement in the sample, adequate data was obtained for 19,242 days and 18,520 nights. We define the difference between true resting heart rate and heart rate when measured at rest across the day (sedentary heart rate). We found that heart rate stabilized in most subjects after 3 minutes of rest. Mean (SD) lowest HR at rest for the sample was 54.5 (8.0) bpm during the day and 50.5 (7.6) bpm at night, and values were highest in the least active group. A circadian variation in RHR was confirmed, with the lowest values in the 24 hour cycle being between 3 and 9 am for most subjects.
Conclusions: Since RHR is such an important clinical metric, it is important for clinicians and researchers to be clear on the measurement methodology used. RHR as measured using a PPG based devices significantly different at night than in the day. A circadian rhythm of RHR was confirmed.
Methods: 433 healthy subjects wore a wrist device that measured activity and heart rate, day and night, for up to 3 months. Heart rate at rest (sedentary heart rate) was recorded, the duration of rest needed for RHR to stabilize was ascertained, and the true lowest RHRs for the sample population during the day and the night were established. The variation of RHR through the 24 hour cycle was also assessed. The sample was also subdivided according to their daily activity levels for subset analysis.
Results: Of the 26,758 days and nights available for measurement in the sample, adequate data was obtained for 19,242 days and 18,520 nights. We define the difference between true resting heart rate and heart rate when measured at rest across the day (sedentary heart rate). We found that heart rate stabilized in most subjects after 3 minutes of rest. Mean (SD) lowest HR at rest for the sample was 54.5 (8.0) bpm during the day and 50.5 (7.6) bpm at night, and values were highest in the least active group. A circadian variation in RHR was confirmed, with the lowest values in the 24 hour cycle being between 3 and 9 am for most subjects.
Conclusions: Since RHR is such an important clinical metric, it is important for clinicians and researchers to be clear on the measurement methodology used. RHR as measured using a PPG based devices significantly different at night than in the day. A circadian rhythm of RHR was confirmed.
