[Background] The circadian rhythms (CR) phase is delayed in young people, meaning that some individuals are unable to keep pace with their school-life schedules and therefore cannot perform to full potential. Conventional methods of measuring CR is not suitable for daily monitoring purposes due to their highly invasive nature and requirement for limited daytime exercise. In this study, we focused on the fact that heart rate decreases to the lowest point during sleep, and is therefore less susceptible to external stimulation. To determine the phase shift in the daily CR of young people, we thus proposed a method for estimating the lowest point of the CR by using electrocardiogram (ECG) during sleep.

[Methods] A total of eight individuals were subjected to analysis (age 22.1 ± 0.6 years). ECG and core body temperature (reference data) measurements were taken during sleep. After analyzing by using our proposed method, we calculated the absolute time differences between the lowest point of the estimated CR by using ECG and measured CR by using core body temperature (error time) to determine any correlations. Further, we investigated the relationship between the usual sleep/wake time and time during the lowest point of the estimated CR.

[Results] Experimental results showed that our proposed method for estimating the lowest point of the CR was highly reliable (error time: 14.1 ± 10.1 min, correlation coefficient: 0.79 ± 0.12). These findings were obtained by comparing the estimated CR taken by using ECG with the reference CR derived by using core body temperature measurement.

[Conclusions] We confirmed both the practicability and effectiveness of estimating the lowest point of the CR by using ECG during sleep. The application of a system that implements the ECG through wearable devices may help improve daily health and can also be used to support clinical studies aimed at circadian sleep disorders.