Exploratory study using wearable devices: Heart rate variability during sleep may be related to depressed mood 

TechDoctor Inc. (CEO: Kazumichi Minato, Headquarters: Chuo-ku, Tokyo; hereinafter "TechDoctor") presented research results on exploratory research for digital biomarkers* using wearable devices for bipolar disorder patients at the 22nd Annual Meeting of the Japanese Society of Mood Disorders held on July 11-12, 2025 (Friday-Saturday) (general session, poster presentation).

This research was conducted to explore the potential use of daily biometric data for monitoring depressive and manic states in bipolar disorder. In this study, in one case diagnosed with bipolar disorder, a tendency was confirmed for RMSSD (Root Mean Square of Successive Differences), one of the heart rate variability (HRV) indicators during sleep, to decrease prior to increases in depressed mood (DM) scores. These results provide exploratory findings regarding mood state change prediction using wearable devices.

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BACKGROUND

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Bipolar disorder is a psychiatric illness that alternates between depressive and manic states, requiring continuous status understanding for symptom management. However, clinical settings often rely on subjective evaluations by individuals,and establishing objective and real-time monitoring methods has been a challenge.Against this background, this research examined the relationship between biometric data such as heart rate, sleep, and activity levels obtained by Google Fitbit Charge6 and subjective mood evaluations using the symptom recording app eMoods.

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RESEARCH OVERVIEW

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Title: Exploration of digital biomarkers using wearable devices in bipolar disorder patients Implementation Institution: TechDoctor Inc.

Research Period: February - November 2024
Subject: One male in his 40s diagnosed with bipolar disorder
Objective Data: Heart rate, sleep, activity level obtained from Google Fitbit Charge6
Subjective Evaluation: Scores from eMoods app
Daily self-evaluation recorded on a 4-point scale from 1 (none) to 4 (severe)
for the following four mood states:
- Depressed mood (DM)
- Elevated mood
- Irritability
- Anxiety

In this research, the temporal relationships between these subjective scores and biometric data obtained from wearable devices were analyzed. The results yielded findings suggesting the predictability of mood fluctuations in bipolar disorder.

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RESEARCH RESULTS

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Regarding RMSSD, one of the HRV indicators during sleep, there were 14 days when values were clearly lower compared to the average over the entire observation period.** On 12 of those days, depressed mood (DM) self-evaluation increased within one week afterward. During the observation period, the subject recorded mood states without knowing the HRV indicator results.

**For each day's RMSSD, the individual mean and standard deviation during the observation period were calculated. Deviations of 1.5 standard deviations or more from the mean were extracted as "abnormal values."

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SIGNIFICANCE AND FUTURE PROSPECTS

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Although this research is an exploratory examination with N=1, it demonstrated the possibility that HRV indicators obtained from routinely acquired wearable data may be related to subjective mood evaluations.

In the future, we plan to conduct verification targeting more cases and continue exploration of relationships with symptom indicators other than depressed mood (DM).TechDoctor will continue to work on developing digital biomarkers contributing to objective monitoring and disease prediction in psychiatric disease areas including bipolar disorder, and engage in research toward realizing personalized medicine.
This research was conducted with approval from the ethics review committee and consent obtained from the subject.

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REFERENCE INFORMATION

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Conference Name: 22nd Annual Meeting of the Japanese Society of Mood Disorders

Conference Dates: July 11-12, 2025 (Friday-Saturday)

Venue: Hamamatsucho Convention Hall

Official Website: https://www.c-linkage.co.jp/jsmd22/

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ABOUT BIPOLAR DISORDER

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Bipolar disorder is a psychiatric illness that cyclically alternates between depressive and manic states, causing major fluctuations in mood, motivation, and behavior. There are large individual differences in symptom appearance and severity, and preventing relapses and understanding status in daily life are considered important challenges.

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ABOUT TECHDOCTOR INC.

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TechDoctor operates under the vision "Towards an era where data improves well-being," developing and promoting social implementation of "digital biomarkers" that derive health-related insights from daily sensing data collected by wearable devices and other means. We aim to realize AI-driven medicine based on data in collaboration with medical, pharmaceutical, food-related companies,and research institutions.

Company Name: TechDoctor Inc.

Website: https://www.technology-doctor.com/

Founded: June 21, 2019

Location: Kyobashi Ed-Grand 4F, 2-2-1 Kyobashi, Chuo-ku, Tokyo

CEO: Kazumichi Minato

Chief Medical Officer: Keisuke Izumi

Business: Development and operation of digital biomarker development platform

"SelfBase," provision of digital medical solutions

* About Digital Biomarkers

Digital biomarkers are indicators for continuously and objectively evaluating the presence or absence of disease, changes in disease status, and treatment effects based on daily biometric data obtained from smartphones, wearable devices,and other sources.

While traditional biomarkers were primarily "point data" measured temporarily at medical institutions, digital biomarkers are characterized by the ability to continuously acquire "line data" from daily life. Based on indicators such as exercise, sleep, and heart rate, they are expected to be used for early disease detection, treatment monitoring, and as new endpoints in drug development.
Development has been progressing overseas since around 2019, and attention is also increasing in Japan.