Measure by measure: Resting heart rate across the 24-hour cycle

Area of Science:

• Cardiovascular physiology
• Biomedical engineering
• Wearable technology

Background:

• Resting heart rate (RHR) is a key health biomarker, but lacks standardized measurement criteria.
• Photoplethysmography (PPG) sensors in wrist-worn devices offer continuous, real-world HR monitoring.
• This study addresses the need for a defined RHR measurement protocol.

Purpose of the Study:

• To establish a working definition and accurate measurement method for RHR using wrist-worn PPG devices.
• To investigate the circadian variation of RHR over a 24-hour cycle.
• To determine the optimal duration of inactivity for reliable RHR measurement.

Main Methods:

• 433 healthy subjects wore wrist devices measuring activity and HR for up to 3 months.
• HR during inactivity was recorded to determine stabilization time.
• Lowest daily HR (true RHR) and its timing were identified, with analysis stratified by activity levels.

Main Results:

• HR stabilized within 4 minutes of inactivity for most subjects.
• Mean RHR was 54.5 bpm during the day and 50.5 bpm at night.
• Lowest RHR values occurred between 0300-0700 hours, particularly in less active individuals.

Conclusions:

• RHR measured by wrist-worn PPG devices exhibits significant diurnal variation, being lower at night.
• A circadian rhythm in HR during inactivity is confirmed.
• A minimum rest period of 4 minutes ensures reliable RHR measurement, best taken between 0300-0700 hours.