Life is not always sunny, and we regularly experience various negative feelings such as stress, anxiety, and ruminative thoughts. These subtle yet accumulative feelings and thoughts could significantly affect long-term well-being and health outcomes [1,2]. Thus, unobtrusively detecting such episodes in daily life and applying low-threshold brief interventions in the moment of highest need (ie, just-in-time adaptive interventions [JITAIs]) [3-5] could prove a highly effective treatment and prevention strategy for an increasingly stressed population. Contrary to so-called pull interventions necessitating an individual’s motivation to request an intervention when needed, push interventions aim to automatically identify moments of highest need (eg, via wearables) and deliver the appropriate intervention [6]. Thus, we aimed to validate JITAIs designed to reduce stress and perseverative cognition (ruminative thoughts) as well as autonomic dysregulation by leveraging transient nonmetabolic decreases in cardiac vagal regulation (so-called additional heart rate variability reductions [AddHRVr]) [7,8].

Heart rate variability (HRV), and particularly the root mean square of successive differences (RMSSDs) metric of HRV, is considered to reflect vagally mediated HRV (vmHRV) and could thus constitute a particularly sensitive tool to identify such episodes of psychophysiological vulnerability, because it signifies a complex interplay between the autonomic nervous system and the central nervous system [9-11]. Specifically, the vagus nerve, as the primary parasympathetic nerve and major constituent of HRV, ensures rapid communication between the brain and the heart (approximately 200 ms), with afferent fibers (from the heart to the brain) outweighing efferent fibers (from the brain to the heart). Hence, vmHRV could signal cognitive function, emotion regulation, and states of stress and psychophysiological vulnerability, among others [10,12,13]. Thus, analyzing the rhythm of the heart throughout everyday life situations may inform about the beneficial or compromised psychosocial functioning of an organism in an ever-changing environment. Without the need for excessive, time-consuming, and burdensome self-reports of perceived stress, letting autonomic regulation decide about the right moment of potential need, the studies in this report aimed for a feasible yet efficient tool to inform JITAIs. Hence, nonmetabolic HRV reductions (ie, AddHRVr) were used to trigger JITAIs, addressing both physiological and psychological indices of well-being.

Recently, we proposed a simulation-based approach to determine individualized trigger settings for AddHRVr and assess their sensitivity for psychosocial states [14]. This approach is based on RMSSD as a measure of vagally sensitive HRV and involves 2 steps (for an overview, please refer to Multimedia Appendix 1 [15]). First, a calibration period aims to derive personalized regression parameters for the algorithm. Specifically, using 1-minute segments of data across a day (12 h), RMSSD is regressed on bodily movement for each individual. The algorithm then uses individualized RMSSD thresholds (>0.5 SD) to identify segments with substantial RMSSD reduction compared to predicted values derived from the calibration. Such AddHRVr (ie, nonmetabolic) [7,8] are tracked in the following days, and a trigger is emitted when a predefined number of AddHRVr occur within a specific time interval [15]. Because AddHRVr is a metric provided every minute, and short-term reductions in RMSSD independent of bodily movement could signal diverse psychosocial states, including positive affect [16], the exact number of segments in a predefined time window is crucial for the validity of an AddHRVr trigger. Notably, in our previous simulation study, varying the analysis window from 2 to 30 minutes and the corresponding threshold from 1 to 29 minutes, a “13 out of 28” algorithm had a power of 0.806 to detect episodes of low social quality interactions, thus suggesting a good performance of this trigger setting [14]. Precisely, when 13 one-minute segments of RMSSD in a time window of roughly half an hour (28 min) were at least 0.5 SD lower than the predicted, given a certain amount of bodily movement (signaling AddHRVr), adverse psychosocial feeling states could be predicted with good power. Hence, we used this algorithm setting (“13 out of 28”) to trigger JITAIs in 2 independent studies. We decided on 2 independent studies to reduce the complexity of the design and allow replication in 2 independent samples.

In study 1, we aimed to use an easy-to-implement brief intervention, which most people can perform quite unobtrusively in various contexts in everyday life. Specifically, slow-paced breathing (0.1 Hz; approximately 6 breaths per min) has been suggested as a powerful and easy-to-learn technique to improve both mental and physical functions [17-21]. In particular, 0.1 Hz breathing has a direct effect on autonomic regulation via strengthening vagal efference and stimulating the baroreceptor reflex, which also positively affects higher central nervous system functioning [10,20]. In previous research, we evaluated acute and chronic psychophysiological effects of slow-paced breathing interventions and demonstrated beneficial effects on both psychological and physiological function [10,22-24]. Other research [20,25] reported a reduction in anxiety and perceived stress following 0.1 Hz breathing, although experimental research could not confirm significant effects for acute stress reactivity [26]. Beneficial effects of slow-paced breathing have also been reported for individuals with hypertension and cardiovascular disorders, thus confirming its clinical usefulness [27,28]. Taken together, slow-paced breathing at 0.1 Hz could benefit both autonomic regulation and psychological health. It is easy to learn and can be applied unobtrusively in various daily life contexts (eg, during work, leisure time, or travel) via smartphones [29,30].

Of note, previous research demonstrated that brief mindfulness (breathing) exercises might be effective in increasing HRV [31-34] and improving well-being via reducing distress and rumination [35,36]. Mindful breathing refers to a technique that aims to facilitate staying in contact with (ie, directing attention to) one’s own breath in a nonjudgmental way. Notably, the design of study 2 allowed it to extend study 1 by including an active control condition. Thus, we could compare the effects of brief mindfulness breathing exercises with external attention exercises, both following either an AddHRVr or a random trigger. We predicted that mindful breathing exercises, when triggered by AddHRVr, are more effective than external focus interventions with respect to perceived stress, ruminative thoughts, and HRV.

In study 1, we aimed to examine whether a 0.1-Hz breathing JITAI would be particularly beneficial when autonomic function is compromised as indexed by AddHRVr. We used a microrandomized trial to compare the effectiveness of AddHRVr-triggered and randomly triggered interventions. Precisely, participants in this preregistered study [37] performed breathing exercises of 1 minute contingent on a prompt on their smartphone. We hypothesized that perceived stress and ruminative thoughts would diminish more strongly following AddHRVr (vs random)–triggered interventions and that HRV (specifically, RMSSD) would increase more strongly during and following the AddHRVr-triggered slow-paced breathing intervention as compared to random prompts.

Likewise, in study 2, we aimed to examine whether brief attention focus exercises are similarly useful for reducing perceived stress, ruminative thoughts, and cardiac autonomic regulation. Specifically, we used a microrandomized design to compare the effects of an internal (respiration) focused mindfulness intervention and an external focus intervention, each triggered either by AddHRVr or randomly. We hypothesized that mindful breathing exercises would decrease perceived stress and rumination and increase HRV relative to an external attention exercise.

Both studies were approved by the local ethics committee (study 1: GZ. 39/93/63 ex 2022/23 and study 2: GZ. 39/54/63/ ex 2023/24) and preregistered via Open Science Framework (study 1 [37] and study 2 [41]). Written informed consent was obtained for both studies, and trials were registered at the German Clinical Trials Register (study 1: DRKS00035684 and study 2: DRKS00035685). Participants were explicitly informed about the psychological concepts of the studies, study duration, and the concepts of JITAIs, as well as the interventions applied. They agreed that data would be analyzed in summary form and used for scientific purposes only (including publication). They were informed that they could withdraw participation at any time without giving reasons and could request deletion of their data upon request. They were also informed to get information in case ectopic beats or arrhythmias were detected to allow clarification by a cardiologist. Data were pseudoanonymized via a personal code not attached to names or other identifying information. Participants received no financial compensation for participation but could receive course credit if applicable.

In study 1, participants received, on average, 45 (SD 8.38; range 31-65) prompts (total n=2571), of which 1782 (69.31%) were answered, 99 (3.85%) were dismissed, 477 (18.55%) were ignored, and 194 (7.54%) were incomplete. Notably, 33% (20/60) of the participants received AddHRVr triggers, while 67% (40/60) did not. On average, 4.14 (SD 7.10) AddHRVr triggers were emitted with an interindividual range between 0 and 26. In study 2, participants received, on average, 43 (SD 11.97; range 24-71; total n=2174) prompts, of which 1307 (60.12%) were answered, 163 (7.49%) were dismissed, 626 (28.79%) were ignored, and 29 (1.33%) were incomplete. Of note, 12% (6/50) of the participants received no AddHRVr triggers, while the remaining participants (44/50, 88%) received between 1 and 39 AddHRVr triggers. On average, 14.41 (SD 10.72) AddHRVr triggers were emitted per individual, which was considerably more than in study 1.

To evaluate the effectiveness of the JITAIs, we applied multilevel (mixed effects) models predicting perceived stress, rumination, and RMSSD, respectively, by AddHRVr trigger (vs random trigger) and time (before intervention vs intervention and before intervention vs after intervention) and the interaction of these variables. For study 2, intervention type (mindful breathing vs external focus) was additionally added to the models. We used RMSSD as the core outcome variable of HRV because it has been particularly recommended in previous ambulatory research [53] and represents the most often studied ambulatory marker of vmHRV [54-56]. RMSSD was quantified as the mean of the 10 minutes before each prompt. Importantly, we controlled for METs as a covariate (mean of 10 min), because the breathing intervention could, in principle, be performed in any body position and independent of bodily movement (eg, during walking or standing or sitting still). RMSSD was analyzed in a time frame of 10 minutes before the intervention (coded as –1), during the intervention (coded as 0), and the following 10 minutes (coded as 1). Of note, to evaluate effects during the interventions, we applied a time frame of SD 2 minutes to account for potential inaccuracy of timing between smartphones and EcgMove4 devices.

To evaluate the robustness of intervention effects on HRV, we also analyzed HF-HRV and SDNN [52]. Specifically, HF-HRV is considered a frequency-domain indicator of vagally sensitive HRV, and SDNN indicates overall autonomic regulation of cardiac activity. To compare adherence with the 0.1 Hz breathing interventions during both types of triggers (AddHRVr vs random) in study 1, we also analyzed low-frequency HRV (LF-HRV; 0.04-0.15 Hz) during the interventions. It should be noted in this respect that respiratory sinus arrhythmia (ie, the fluctuation of heart rate concordant with breathing) shifts to the LF-band (0.04-0.15 Hz) during breathing with 6 breaths per minute (corresponding to 0.1 Hz) [20]. HRV measures were logarithmized (ln) before analysis to account for skewness.

The interaction between prompt (random vs AddHRVr) and time (before intervention vs intervention and before intervention vs after intervention) would indicate the effectiveness of the AddHRVr-triggered intervention on the outcome variables mentioned earlier. For analyzing study 2, we also centered intervention type within persons. Throughout the analyses, we additionally controlled for age, gender, BMI, METs, and momentary smoking. Since age, gender, and BMI did not change any of the associations of interest, they were removed from the final models. Continuous predictor variables were centered. Moreover, we centered trigger and intervention (study 2) on the within-person level to allow interpretation of within-person effects [57].

Analyses were conducted using R (version 4.4.0 [48]) with the package lme4 (version 1.1-35.3 [58]). Note that b refers to the unstandardized estimate of the relationships. We fixed the level of significance at P<.05 (2-tailed).

In both studies, we aimed to evaluate the effectiveness of physiologically triggered (ie, AddHRVr) JITAIs to restore psychological well-being and autonomic flexibility. In study 1, we used a brief (1 min) slow-paced breathing intervention, and in study 2 we applied brief mindful breathing and external focus exercises. The findings document that such 1-minute interventions resulted in a general decline of stress and rumination, independent of the right moment. Both slow-paced breathing and mindful breathing interventions applied at the moments of compromised cardiac vagal function (ie, following AddHRVr triggers) resulted in a stronger increase or renewed increase of HRV. Precisely, we observed higher HRV during the breathing interventions and up to 10 minutes thereafter when the intervention was triggered by AddHRVr as compared to randomly triggered prompts. It should be mentioned in this respect that in study 2 the renewed increase in HRV following the interventions could also be observed for the external focus exercises, thus suggesting a rather nonspecific intervention effect in reestablishing autonomic flexibility.

Importantly, AddHRVr contingent increases in HRV during and after the interventions could be verified for different indices of HRV (namely RMSSD, HF-HRV, and SDNN), thus suggesting rather broad-bended beneficial effects on cardiac autonomic regulation. Hence, triggering brief interventions in moments of low HRV could prove particularly beneficial for stabilizing cardiac autonomic regulation, thus arguing for the just-in-time intervention concept. Importantly, the cardiac vagal rebound relative to preintervention levels observed in both studies was more pronounced than could normally be expected without interventions. Specifically, analyzing responses to so-called virtual AddHRVr triggers (nonmetabolic HRV decreases that did not evoke triggers but showed the same characteristics as the AddHRVr triggers) suggested that the increase in HRV following the AddHRVr-triggered interventions was significantly more pronounced than the natural increase of HRV following virtual AddHRVr triggers, thus excluding the interpretation of spontaneous rebound effects. Thus, it seems that shifting attention to either internal or external cues or engaging in slow breathing exercises was a main driver for the increase in cardiac autonomic flexibility.

The findings of study 1 generally align with research suggesting beneficial effects of slow breathing on emotional valence, perceived stress, and anxiety in controlled settings [18,59,60]. Moreover, the finding confirms research showing that slow breathing biofeedback increased attentional control in individuals reporting higher levels of stress [61], suggesting that slow breathing exercises are particularly useful when applied during elevated (physiological) stress. Moreover, the conceptual replication in study 2 confirms research on the beneficial effects of mindful breathing for mental health [35,36,62]. However, we need to emphasize that, similar to study 1, in contrast to the HRV effects, the psychological effects for stress and rumination were evident irrespective of the cardiac autonomic state and, moreover, independent of the type of intervention. This result aligns with research suggesting, for example, beneficial psychological effects of breathing exercises irrespective of the pace [63].

Because any of the interventions applied seemed to benefit both rumination and perceived stress independent of the cardiac autonomic state, common mechanisms inherent in both interventions need to be discussed. First and foremost, it could be assumed that both interventions facilitated distraction from ongoing activities. Distraction has been considered a powerful emotion regulation strategy [62], especially when combined with acceptance [64]. Notably, there is also neurobiological evidence that distraction as an emotion regulation strategy is associated with a comparably strong decrease in amygdala activity, thus substantiating the beneficial effects on subjective well-being [65]. Within a broader perspective, attentional deployment has been considered crucial for emotion regulation, for example, to either distract oneself from negative cues or refocus on positive cues [66]. Taken together, we would assume that the psychological effectiveness of the JITAIs applied in this research was mainly driven by the deployment of attention.

The studies had some limitations that need to be acknowledged. First and foremost, they failed to show that perceived stress and rumination could be reduced more strongly when triggered by HRV. In fact, irrespective of the trigger, both stress and ruminative thoughts were ameliorated in the course of the interventions. Consequently, unlike HRV, the psychological variables do not strongly argue for a just-in-time approach. Certainly, more research is warranted to examine the exact circumstances under which participants will benefit more strongly with respect to their psychological well-being. Alternatively, refining the trigger settings to detect psychologically vulnerable periods in time with higher accuracy and adapting the interventions to the specific needs constitutes an important task for future research [67].

Second, sample sizes for both studies were rather moderate, and the general levels of perceived stress and rumination were rather low. Future research should increase both sample sizes and days of assessment and assess more heterogeneous samples with compromised well-being to uncover effects with higher sensitivity, especially the superiority of AddHRVr-triggered interventions for perceived stress and rumination.

Third, there was no passive control intervention, thus limiting the interpretation of the findings. Thus, we could not rule out the possibility that any kind of alternative activity or disruption of everyday life activities for 1 minute may benefit well-being and cardiac autonomic regulation. Accordingly, study 2 suggests that effects on psychological variables might indeed be unspecific to the intervention applied. Relatedly, we cannot rule out that the reduction of perceived stress and rumination indicates mere order effects or psychological adaptation to the repeated assessment or even socially desirable responding. Taken together, a control group without interventions or a within-person condition without intervention is needed in future research to ascribe the effects solely to the interventions.

Fourth, while we analyzed immediate and delayed cardiac autonomic effects of the interventions, it should be kept in mind that the analyzed time window for the postintervention effects was restricted to 10 minutes. It would be worthwhile to analyze longer time scales to examine more sustained effects of the 1-minute interventions. Fifth, in study 1, only about 33% (20/60) of the participants received AddHRVr triggers, thus suggesting that either the trigger setting was too conservative, ultimately missing less severe episodes of cardiac vagal reductions in most participants, or calibration was not sensitive enough to identify episodes of psychophysiological vulnerability. Notably, the same trigger settings were deemed more suitable in study 2, with 88% (44/50) of the participants receiving AddHRVr triggers. We are not sure about the cause of this discrepancy. However, it should be noted in this respect that the performance of the trigger depends on the generalizability of the 12-hour calibration period. If physical activity changes substantially after the calibration period, the algorithm settings may become inaccurate, leading to less reliable triggers. Future research is advised to aim for a representative day for calibrating the algorithm or to use more adaptive trigger settings, for example, via implementing dynamic algorithms [67], adjusting the intercept, which could account for shifts in autonomic regulation toward higher or lower levels of RMSSD. Furthermore, easier-to-handle algorithms might allow us to scale up the application of JITAIs by means of commercially available wearables.

Finally, although this research indicated that JITAIs triggered by nonmetabolic decreases of HRV are promising to intervene at the right moments and have convincing proximal effects, their long-term (distal) impact on physical and psychological well-being is yet unclear and calls for further research regarding their superiority relative to traditional intervention regimens. For example, it would be important to know if such brief interventions, when conducted regularly over a longer period, would have the potential to decrease risk for cardiovascular or mental diseases beyond the effects achieved by traditional intervention regimens.

In this research, we demonstrated that applying brief, slow-paced breathing or mindful breathing interventions in everyday life via smartphones, triggered at moments of autonomic vulnerability, seems to stabilize mental health and cardiac autonomic function up to 10 minutes following the interventions, thus aligning with previous research conducted in traditional, standardized settings [10,19,20,23,36,59]. To the best of our knowledge, these are the very first microrandomized trials using an AddHRVr algorithm to automatically identify moments of psychophysiological vulnerability and trigger brief interventions (for a similar approach using physiological data, refer to the study by Hovsepian et al [68]). The findings are particularly important as they suggest that intervening at moments of compromised HRV could immediately boost cardiac autonomic function, and any kind of intervention could prove successful in restoring HRV up to 10 minutes to a certain degree. The findings were not restricted to a specific HRV metric but showed evidence toward a generalized increase in cardiac autonomic flexibility, thus suggesting clinical relevance. As speculated earlier, distraction or attentional deployment might be the core driver of these effects. In a similar vein, the general relieving effect on perceived stress and rumination independent of the trigger and type of intervention may signal a time-out effect, potentially restoring well-being.

The concept of JITAIs has raised considerable interest in recent years, and given the formidable growth and availability of wearables, digital mobile technologies, and algorithm developments, the field will continue to prosper [69]. However, research is yet in its infancy, and future studies need to convincingly demonstrate that JITAIs are superior to traditional intervention regimens and worth the tremendous effort in setting up algorithms sensitive and specific to certain psychophysiological states (eg, stress). In this respect, the adaptive part of JITAI needs is yet to be established. The proposed approach in this research did not adapt interventions to the specific needs but mainly followed the just-in-time concept.

Frequent episodes of adverse psychological states, such as stress, could impact health and well-being in the long run. Identifying such moments of vulnerability could be useful to directly counter such states via microinterventions to ameliorate their health impact. However, the usefulness of such interventions needs solid empirical research. In the 2 studies, we demonstrated that brief psychological exercises of 1-minute length, applied in moments of compromised autonomic flexibility (low nonmetabolic HRV), restored autonomic function and benefited psychological well-being. The findings suggest that brief psychological interventions can be implemented in daily life via wearables, possibly promoting mental and physical health longitudinally.