Diverse mental challenges are a major part of daily life, and the flexible adjustment to these day-to-day demands seems mandatory for successful coping [1]. However, this adaptive ability of self-regulation greatly varies between people and situations. Notably, mental effort, stress, and self-regulation are indexed by functions of the vagus nerve as the primary part of the parasympathetic nervous system [1–5]. A putative physiological marker of vagal control is the beat to beat variation in heart rate, the so-called heart rate variability (HRV) [6,7]. The vagal influence on HRV can be quantified in the time domain by means of RMSSD (root mean square of the successive difference in normal sinus beat intervals) and pNN50 (percentage of adjacent normal sinus beat intervals that differ by more than 50 ms) [8–10].

While the tonic HRV level potentially indicates an inter-individual trait like capacity to adapt to environmental challenges by parasympathetic/vagal control [11–13] (for a meta-analysis see [14]), task-related HRV changes represent the dynamic task-specific vagal control of the heart (i.e., vagal break) [2,15]. Therefore, task-related HRV is an even more sensitive variable in the context of regulatory efforts of the organism and the actual flexible adaption to physical and mental challenges [2,3,6,7,13,16,17]. Following from this, the adjustment of the organism to task-related challenges—e.g., self-regulatory efforts [4,13]—may be best represented by the flexible change of the HRV from a baseline to a period of mental effort.

However, not every mental challenge requires the same amount of cognitive and regulatory capacity [18,19]. Of note, while many mental tasks are associated with task-related HRV decreases [16,20,21], there is evidence that some cognitive tasks are associated with task-related increases [3,4,15]. For instance, Butler et al. [13] found HRV increases during the reappraisal of negative events [22,23], and Silvia et al. [24] reported a slight rise of vagal control during a creative ideation task. This is remarkable, since creative ideation and cognitive reappraisal (the latter as an instance of “creativity in an affective context” [25]) represent complex self-regulatory cognitive processes (e.g., flexible switch between associative and executive functions), which share important basic cognitive functions [25–28].

A unique characteristic of creative and divergent thinking tasks in contrast to traditional convergent cognitive tasks is the interplay of associative/spontaneous modes of thinking and executive top-down control processes [29–34]. Following dual process theories, creative ideation is not solely based on mental effort, which is associated with HRV decrease [24,35–38], but also on spontaneous, automatic, associative, and relaxed modes of thinking, which are presumably associated with HRV increase [39–42]. Furthermore, the flexible shift between these different modes of thinking is important for creative ideation performance [43–45]. The contribution of both modes of thinking to creative ideation may explain contradictory findings in literature. Loudon and Deininger [35] reported a negative and Bowers and Keeling [40] a positive association between indices of creative performance and HRV. Silvia et al. [24] found no significant link between task-related vagal control and originality.

However, a further reason for inconsistent findings in this context might be that cognitive performance and HRV are associated with the amount of regular physical exercise, physical fitness, and general health of people [6,8,46] (for cognition see [47,48]; but see [49]; for cognition and HRV see [50]). Furthermore, in a recent study, Latorre Román et al. [51] found an association between measures of physical fitness and creativity (see also [52,53]; similar findings for acute physical activity see, [54–56]). The cross-stressor adaptation theory [57] assumes that salutary cardiovascular reactivity and recovery due to physical training generalize from ergogenic challenges to psychogenic challenges. This generalization may lead to a differentiation of cardiac reactivity to psychological stressors between people doing more and people doing less exercise and people who might therefore be more or less physically fit [46,58–60].

Following this idea, the present study examined whether regular exercising may moderate the association between task-related HRV changes and the performance outcome in a creative ideation task. People regularly exercising more are supposed to be physically best prepared to show adaptive task-related changes of HRV [6], in order to flexibly match the adaptive states important for optimum performance outcome [1]. More precisely, it was assumed that participants with a higher amount of regular exercise may more flexibly adjust to the creative challenge and more easily switch between mental effort [24,35] and spontaneous and automatic modes of thinking [40–42]. Therefore, it was hypothesized that participants exercising more often will show higher task-related HRV during creative ideation in contrast to participants exercising less, and that in exercisers (but not necessarily in sedentary people) the HRV response will be related to task performance.

The aim of this study was to examine the interplay of regular physical exercise and HRV reactivity during a challenging creative ideation task on task performance. Generally, HRV tended to decrease during the task, particularly at the beginning, which indicates the mobilization of mental effort and is characteristic of challenging performance tasks [87,88]. However, participants greatly varied in their cardiac response to the challenge. While some showed marked decreases of HRV, others exhibited smaller decreases or even increases of HRV during the task [15]. This variability in the cardiac responses was meaningfully related to task performance in terms of the originality of generated creative ideas. However, only when inter-individual differences in regular exercising were considered.

In people with a higher amount of regular exercise, greater HRV increase (i.e., vagal activation) during the creative ideation task was correlated with greater originality of the produced ideas, which is in accordance with some relevant research [40,41]. By contrast, in more sedentary people this association was reversed. Greater task-related HRV decrease (i.e., greater vagal withdrawal) was associated with better task performance in terms of originality, which is in line with other studies in the literature [35]. Importantly, regular exercising as such was not correlated with task performance, that is, on average, people exercising both more or less achieved similar levels of originality [52,89]. This null finding might take place because of study-design divergences. Contrary to former studies [51,53], we used a self-report measure of regular physical exercise [52]. Furthermore, while Gondola and Tuckman [53] reported increases in the fluency parameter of the AU task and Latorre Román et al. [51] found a positive effect for a composite score of multiple creative thinking tasks, we operationalized creativity by means of originality of ideas.

Nevertheless, the observed interaction effect suggests that greater originality in a creativity task is not only achieved by the investment of more mental effort [24,35], and more spontaneous and associative modes of thinking [32], but that both can lead to success, depending, among others, on the physical constitution of the person. This finding may in part explain the heterogeneity of previous results on associations between cardiac vagal control and creative performance [24,35,40].

Furthermore, the observed moderation effect at least in part supports assumptions of the cross-stressor adaptation theory [57], according to which changing cardiac response patterns to physical stressors by physical exercising may translate to altered modes of responding of regulation functions when confronted with psychological stressors [46,59,60]. People who generate more original ideas showed smaller vagal withdrawal (or even increases in heart rate variability) during task performance, when they were regularly exercising as compared to being more sedentary. This is in line with Park et al. [90], who reported task-related increases of HRV during a cognitive task in participants with higher flexible regulation functions (i.e., higher tonic vagal control) and decreases in participants with lower regulation functions (for short review see [15]).

The present findings need to be discussed in light of some limitations. First, the amount of regular physical exercise was only assessed by self-report and not by a behavioral index. Although the current findings need to be replicated with objective measures, similar self-reports of exercise have proven suitable to reveal reliable and meaningful associations with challenge-related cardiac responses [59]. Second, it might be proposed that HRV is influenced by verbal responses during the creative ideation task [91]. However, even if the vocalization of ideas has an impact on HRV, this cannot explain the reported associations between HRV and originality (for people exercising more or less, respectively), since the number of generated ideas (within the 6 minutes activation period) was virtually independent of task performance (originality). This is further in line with the result, that the respiration frequency (EDR) had no significant impact on the findings of this study. Third, although the eye closed baseline condition is the most frequently used approach in psychophysiological research [15], an additional baseline using an eyes open condition might have been valuable to evaluate the specificity of reported task-related HRV changes. Fourth, due to the cross-sectional, correlational nature of the present study, no cause-effect relationship can be deduced from the results.

Despite of these limitations, the findings of this research suggests that participants exercising more often in their daily life achieve higher originality with more flexible, associative, and spontaneous modes of thinking, while participants exhibiting a more sedentary lifestyle more strongly invest mental effort to reach the same quality level of produced ideas. Since solving open and ill-defined problems constitutes a daily mental challenge for many people such mechanisms might impose health risks and lower well-being in people with a sedentary lifestyle in the long run. Conversely, grounding on the cross-stressor adaptation theory the findings of the present research supports the beneficial effect of regular exercising on mental challenges in daily life [46,57].