Over the past decade, wearable technology has gained significant traction, with smartwatches emerging as one of the most popular and widely adopted devices in this category. A smartwatch is a wrist-worn device equipped with various sensors, connectivity features, and a display screen, offering functionalities beyond traditional timekeeping [1]. These devices have seen exponential growth in popularity due to their versatility, convenience, and ability to seamlessly integrate with smartphones and other smart devices. Smartwatches initially gained attention for their fitness and activity tracking capabilities. They provided users with the ability to monitor their physical activity, track steps, measure heart rate, and calculate calorie expenditure [2]. The incorporation of advanced sensors, such as optical heart rate sensors and accelerometers, allowed users to gain insights into their health and wellness in real time. Beyond fitness tracking, smartwatches quickly expanded their features and functionalities to include communication, notifications, mobile apps, music playback, and more [3].

As the capabilities of smartwatches advanced, manufacturers recognized the potential to address another pressing issue affecting individuals’ well-being: stress management. The detrimental effects of chronic stress on physical and mental health have led to a growing demand for tools and techniques that help individuals monitor and alleviate stress levels [4]. This is where the integration of heart rate variability (HRV) analysis into smartwatches has garnered attention. HRV, which measures the variation in time intervals between consecutive heartbeats, is an indicator of autonomic nervous system activity ^[5][6][5,6]. It has been widely studied and recognized as a valuable metric for assessing stress levels, emotional states, and overall well-being. By exploiting optical sensors and advanced algorithms, smartwatches can capture and analyze HRV data, providing users with insights into their stress levels and offering interventions to manage and reduce stress effectively ^[7][8][7,8]. The integration of stress management features based on HRV analysis has positioned smartwatches as holistic wellness devices [8]. By combining fitness tracking, communication, and stress management capabilities, these devices have the potential to empower individuals in maintaining a healthy lifestyle and have emerged as a promising solution at the intersection of technology and personal health.

Heart rate variability (HRV) refers to the fluctuation in the time intervals between consecutive heartbeats, also known as R-R intervals, as measured by electrocardiography (ECG) or optical sensors ^[5][6][5,6]. It reflects the dynamic balance between the sympathetic and parasympathetic branches of the autonomic nervous system (ANS), which regulates our body’s physiological responses to stressors [9]. The ANS plays a crucial role in modulating stress by regulating heart rate, blood pressure, respiration, and other vital functions. The sympathetic branch of the ANS is responsible for the “fight-or-flight” response, activating the body to cope with stress, while the parasympathetic branch promotes relaxation and restoration ^[3][10][3,10].

When individuals experience acute or chronic stress, the sympathetic branch of the ANS becomes dominant, leading to increased heart rate and decreased HRV. Conversely, during periods of relaxation and recovery, the parasympathetic branch prevails, resulting in decreased heart rate and increased HRV ^[9][11][9,11]. Therefore, a higher HRV is generally associated with a more adaptive stress response and better overall well-being ^[9][12][9,12].

HRV analysis provides valuable insights into an individual’s physiological state, including their stress levels, emotional states, and autonomic balance [11]. By continuously tracking HRV throughout the day, these devices can provide real-time feedback on stress levels and suggest personalized interventions to help users regulate their stress response [8]. These interventions may include breathing exercises, guided meditations, mindfulness prompts, or activity recommendations tailored to everyone’s needs. Furthermore, comparing HRV patterns before and after implementing stress reduction techniques, individuals can objectively assess the impact of different strategies and make informed decisions about which methods work best for them and help them to enhance their self-awareness, adopt healthier coping mechanisms, and ultimately lead a more balanced and stress-resilient life ^[13][14][13,14].

2. HRV and Stress Management

Heart rate variability (HRV) relates to the variation in the time intervals between successive heartbeats. It is a physiological phenomenon caused by the balance of the sympathetic and parasympathetic branches of the autonomic nervous system. Heart rate variability (HRV) has gained a lot of interest as a stress and overall well-being metric. Here is a quick rundown of heart rate variability (HRV) as a stress indicator: The autonomic nervous system (ANS) controls various involuntary bodily functions, including heart rate. The sympathetic nervous system (SNS) handles the “fight-or-flight” response, whereas the parasympathetic nervous system (PNS) controls the “rest-and-digest” response. HRV depicts the dynamic interaction between these two branches. High and low HRV indicate greater variability in heartbeat intervals, indicating a flexible and adaptable autonomic nervous system (ANS) [11]. It suggests a stronger parasympathetic response and increased stress resilience. Lower heart rate variability (HRV), on the other hand, indicates decreased variability and increased sympathetic dominance, which may be associated with chronic stress, fatigue, or health concerns. Monitoring heart rate variability (HRV) can help with stress management for several reasons: HRV provides information on the physiological response of the organs to stress. Individuals can detect early signs of stress and intervene before they worsen by regularly monitoring their HRV ^[9][15][9,17]. The early detection of stress allows for early intervention and prevention. In addition, HRV monitoring allows for a customized stress assessment. Everyone has a unique baseline HRV, and variations from this baseline can indicate changes in stress levels [9]. By researching their own patterns, people can gain a better knowledge of their stress reactions and develop individualized stress management strategies. HRV biofeedback training comprises learning self-regulation skills with the use of real-time HRV readings ^[16][17][18,19]. Individuals can improve their ability to self-regulate physiological responses by assessing the effects of stressors on HRV and using stress-reduction strategies. Biofeedback training allows people to actively manage their stress levels and achieve a condition of balance and relaxation. Chronic stress can be harmful to both physical and mental health. Monitoring HRV can help people recognize stressors and make healthy lifestyle adjustments. Individuals can improve their stress management, resilience, and overall well-being by actively regulating their stress levels and boosting their HRV ^[9][17][9,19]. HRV monitoring can provide useful information on the efficacy of lifestyle adjustments. Individuals could examine how changes in sleep patterns, exercise routines, food, or relaxation techniques affect their HRV ^[18][19][20,21]. This feedback loop allows for the ongoing optimization of stress management strategies based on individual responses and preferences. As a result, HRV monitoring contributes to a holistic approach to health by taking the mind–body link into account. Stress influences both mental and physical health, and HRV serves as a bridge between the two. By monitoring HRV, people can actively manage stress from all angles and increase their overall health and fitness. The concept of the autonomic nervous system (ANS) and its role in stress response and control can be used to comprehend a theoretical framework that links HRV and stress reduction. The theoretical framework described below describes the relationship between HRV and stress reduction: The ANS controls the body’s reaction to stress. Its two branches are the sympathetic nervous system (SNS) and the parasympathetic nervous system (PNS). In stressful situations, the sympathetic nervous system (SNS) triggers the “fight-or-flight” response, resulting in increased heart rate, blood pressure, and decreased HRV. The parasympathetic nervous system (PNS), on the other hand, activates the “rest-and-digest” response, promoting relaxation, a lower heart rate, and an increase in HRV ^{[3][6][9][10]}[3,6,9,10]. Stress reduction measures can improve HRV. Individuals who adopt these methods activate the parasympathetic nervous system (PNS), resulting in relaxation and improved HRV. Approaches to stress reduction that frequently activate the parasympathetic nervous system (PNS) can result in long-term changes in autonomic nervous system (ANS) balance and improved stress resilience [9]. The connection between HRV and stress reduction results in a positive feedback loop. Individuals who utilize stress-reduction techniques and perceive an increase in HRV receive positive feedback [13]. This feedback loop encourages continual stress-reduction practice, resulting in greater improvements in HRV and stress reduction. Consequently, stress resilience can be promoted by the regular practice of stress reduction techniques as well as subsequent increases in HRV. Individuals who have a healthy ANS response become better equipped to handle stress and preserve emotional well-being ^[20][24]. Increased HRV shows a flexible and adaptive ANS capable of responding to stimuli effectively without excessive physiological arousal [11]. Thus, this theoretical framework provides a conceptual understanding of the link between HRV and stress reduction. Because the actual efficacy and effects of stress reduction approaches on HRV differ from person to person, empirical research and therapeutic support should be used in tandem with this paradigm.

3. Smartwatches and Stress Management

Smartwatches are popular wearable devices to help reduce stress ^[21][25]. 

Most smartwatches monitor the wearer’s heart rate throughout the day. Since stress and anxiety raise the heart rate, smartwatches can show stress levels. Real-time heart rate data may reveal stress causes [8]. Advanced smartwatches analyze HRV. HRV measures the fluctuation in heartbeat intervals to better assess stress and autonomic nervous system balance. HRV-analysis smartwatches may help customers manage stress ^[5][9][5,9]. Many smartwatches track stress using heart rate, HRV, and other metrics. These devices may detect high stress levels and provide real-time reminders to do breathing exercises or mindfulness [8]. Some smartwatches include relaxing or breathing techniques. These features guide users through deep breathing or mindfulness exercises, helping them relax in stressful times. The watches may give visual or tactile feedback to help users follow the workouts ^[22][26]. Sleeping sufficiently reduces stress. Smartwatches measure sleep duration, phases, and quality. Examining sleep patterns may disclose causes of stress and tiredness ^[23][27]. Exercise reduces stress. Smartwatches measure steps, distance, calories, and exercise. Smartwatches may reduce stress by encouraging regular activity and providing feedback ^[24][25][28,29]. Smartwatches can plan mindfulness or relaxation breaks. These reminders may encourage users to take short breaks, breathe deeply, or meditate for stress management and mental health ^[26][30]. Smartwatches also link to smartphone applications or online platforms for extensive data analysis. Users may analyze stress patterns, trends, and historical data to discover triggers and make educated lifestyle adjustments and stress reduction choices ^[27][31]. Smartwatches detect and analyze HRV using optical heart rate sensors and advanced algorithms. Smartwatches assess heart rate variability: Smartwatches’ undersides include optical heart rate sensors that touch the wearer’s skin. LEDs illuminate the skin, while photodiodes detect the reflected light. Photoplethysmography (PPG) is this technique. LEDs send light into the skin, which the blood vessels absorb, and the photodiodes reflect. Photodiodes detect blood-induced light intensity changes. The wearer’s pulse is shown as a PPG signal ^[28][32]. Smartwatches use the PPG signal to calculate heart rate by analyzing the period between heartbeats. Smartwatches display real-time heart rate data ^[29][33]. Algorithms analyze the raw PPG signal and derive HRV data in smartwatches. HRV monitoring relies on variations in pulse intervals ^[30][34]. Frequency-domain HRV analysis is commonplace. The FFT converts the raw PPG signal into the frequency domain. This transformation isolates signal frequency components ^[5][31][5,35]. Frequency-domain analysis determines HRV parameters. High frequency (HF), low frequency (LF), and the LF/HF ratio are factors. High-frequency (HF) power represents the parasympathetic nervous system, LF power represents sympathetic and parasympathetic activity, and the LF/HF ratio indicates sympathetic nerve activity [5]. Smartwatches may evaluate stress using HRV and contextual data. Advanced algorithms and machine learning methods analyze HRV data, compare it to established patterns, and predict stress levels based on the individual’s baseline and deviations ^[7][8][7,8]. Smartwatches and smartphone apps display HRV data and stress assessment findings. HRV trends, stress levels, and stress management suggestions are available ^[32][36]. Smartwatches simplify HRV monitoring; however, their accuracy and precision might vary. Sensor quality, skin contact, motion artifacts, and algorithm design affect HRV data dependability. Clinical-grade HRV analysis may need medical equipment and professional interpretation. Smartwatches are beneficial for stress management, as they have key advantages. First, smartwatches monitor heart rate, HRV, and stress in real time. Awareness helps people recognize stress and reflect on its causes. Recognizing stress patterns helps people reduce stress [8]. Smartwatches provide continuous monitoring without extra equipment or difficult processes. They provide real-time heart rate, HRV, and stress data to users ^[33][37]. Continuous monitoring helps people track their progress, identify triggers, and make stress management changes. Smartwatches track stress patterns over time. Historical data and trends may help people understand their pressures, find patterns, and make educated lifestyle choices and stress reduction decisions ^[33][34][37,38]. These personalized insights may help people design personalized stress management plans. Smartwatches guide users through stress-reduction activities like breathing and mindfulness. Step-by-step instructions, visual cues, and tactile feedback help consumers relax. This guidance emphasizes stress-reduction and stress management [14]. Smartwatches also measure sleep and exercise. A healthy lifestyle includes exercise, sleep, and stress management. Smartwatches encourage healthy behaviors that reduce stress and improve well-being ^[35][36][39,40].