Keywords

1 Introduction

In Japan, the mental health of the general population is receiving increasing attention. A more pertinent statistic here would be to cite statistics regarding mental stress in Japan, such as the prevalence of high stress among workers. Indeed, in December 2015, Japanese companies with more than 50 employees were required by law to test the mental stress levels of employees [1]. Mental stress is typically subjectively screened for using questionnaires. More recently, research has indicated that heart rate variability (i.e., intervals between R waves, or R-R interval, which relates to activation of the autonomic nervous system), salivary amylase, and various other biological data can serve as accurate indicators of mental stress, which has led to the development of methods of testing stress based on these data [2–4].

Mental stress may decrease when workers actively engage with their work. For example, workers who find it difficult to engage in menial work may experience less mental stress when engaging in creative work. Therefore, we consider that metal stress may be related with work type. However, the relationship between work type and biological indicators has not been studied sufficiently—it is not clear how these data differ between work types, such as creative and non-creative work. In this paper, we define the meaning of creative work that persons have to consider method and solution for task by him/herself. Also, we define non-creative work that person does not have to consider method and solution for task by him/herself and is required to perform just according to some rules. Thus, the purpose of this paper was to clarify how creative and non-creative work influence R-R intervals.

2 Experimental Methods

Eighteen individuals aged 21–24 participated in this experiment. Participants were explained ethical guideline and agreed with the contents. Written informed consents were obtained from all participants. Participants were asked to try four tasks (A–D), as shown in Fig. 1. Task A was termed the “marshmallow challenge” [5], and involved having participants build the tallest structure they could using uncooked dried spaghetti noodles; an entire marshmallow had to be held at the top of the structure. Task B was called “binding”, and required participants to assemble triangles by binding dried noodles with kite strings together as much as possible in the time allotted. Task C was “calculating”, wherein participants added together 3-digit numbers, and Task D was “spot the difference”, wherein participants had to identify an anomalous character out of a hundred identical characters. Both tasks had to be performed as much as possible in the time allotted. Task A was considered to require the most creativity out of the four tasks because participants who perform Task A to be required to consider the method and solution by him/herself. Also, Tasks A and B were craftworks requiring manual dexterity, while Tasks C and D were simple, repetitive tasks that did not require creativity.

Fig. 1.
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Task contents

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The participants attempted each task three times per day, as shown in Fig. 2. The time allotted for each task was 30 min in total. First, participants were instructed on how to perform one of the four tasks. Then, the participants took a rest, wherein they sat with both eyes closed for 3 min. After this, the participants performed the task for 6 min as the first attempt. Each attempt comprised two periods of 3 min (former and latter); when the latter period began, a timekeeper began a countdown in 1-min intervals (e.g., “last 3 min”, “last 2 min”, and “last 1 min”). After the 6-min attempt, participants again took a rest with both eyes closed in a sitting position. The participants repeated this procedure for two more attempts. During all task attempts and rests, participants’ R-R intervals were recorded with a wearable heart rate meter (Uniontool, WHS-1).

Fig. 2.
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Example of a timetable. The task order was randomly determined for each individual. The columns depict Periods 1–10 (repeated rests and attempts)

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Upon completing all attempts of the day, the participants responded to a self-assessment questionnaire with the following format:

  • Did you experience a feeling of “stress” during the 1st/2nd/3rd attempt?

Eleven different words appeared within the quotation marks (e.g., “stress”, “fatigue”, “tension”). The ordering of the quoted words changed each day at random. The participants selected one of five responses to the questions, as follows.

  1. 5.

    I very much think so.

  2. 4.

    I think so.

  3. 3.

    I slightly think so.

  4. 2.

    I only slightly think so.

  5. 1.

    I do not think so.

These choices have an ordinal relationship. Figure 3 shows the response format of the five choices.

Fig. 3.
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A portion of the self-assessment questionnaire

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3 Results and Discussion

3.1 Differences in R-R Intervals Between Resting and Work

Figure 4 shows examples of time-series datasets of R-R intervals. The stepped lines in the figure show the median R-R intervals in each period. Figure 5 shows box plots of the medians of all participants; the horizontal axis shows each period.

Fig. 4.
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An example of a participant’s time series of R-R intervals

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Fig. 5.
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Box plots of median R-R intervals

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Figure 4a shows one participant’s time series of R-R intervals in Task A. For example, the median R-R interval in Period 1, wherein the participant took his/her first rest, was 724 ms. Then, the median R-R intervals in Periods 2 and 3, wherein the participant made the first attempt to complete Task A, were 643 and 616 ms, respectively. As can be seen, they were lower than the median R-R interval in Period 1, with differences of 81.0 and 108 ms, respectively. The median R-R interval in Period 4, wherein the participant took his/her second rest, increased to 686 ms. As shown in Fig. 4a, this pattern was observed for the subsequent periods. In contrast, there were no clear differences in R-R intervals between rests and attempts in Task D, as shown in Fig. 4b. The difference in medians between Periods 1 and 2 for Task D was only 8.50 ms, even though the results in Fig. 4a and b are derived from the same participant. We noted these same two patterns depicted in Fig. 4 in some of the participants. The results of Task B were similar to those of Task A, while the results of Task C were similar to those of Task D.

Figure 5a shows distributions of medians in each period across all participants, and is in line with the abovementioned patterns. Figure 5a shows that the median R-R intervals during attempts at Task A tended to be lower than were those during the rests. The Friedman test indicated that the medians significantly differed among the periods in Fig. 5a (P < 0.001). We also noted significant differences in Fig. 5b (P < 0.001). Specifically, participants’ R-R intervals were quicker during their attempts at the craftworks (e.g., Tasks A and B) compared with resting. In contrast, the medians did not significantly differ among the periods in Tasks C and D, as shown in Fig. 5c and d.

Notably, the median R-R intervals in Fig. 5a were lower than were those in Fig. 5c and d, even during the rest (i.e., Period 1) before the first attempt at Task A. For example, the median R-R intervals in the first rest in Fig. 4a and b were 724 and 927 ms, respectively, with a difference between them of 203 ms. Overall, our results indicated that participants’ heart rates did not quicken when attempting simple work such as Tasks C and D.

3.2 Correlation Between Variability of R-R Intervals and a Feeling of Stress

Figures 6 and 7 show the scatterplots of the correlations (Spearman’s rank correlation ρ) between variability in R-R intervals and a reported feeling of stress. In these figures, the horizontal axes show differences in median R-R intervals between Period 1 and the periods when participants attempted the tasks, while the vertical axes showed the strength of the feeling of stress based on questionnaire responses.

Fig. 6.
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Correlations between variability in R-R intervals and feelings of stress in Task A (marshmallow challenge).

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Fig. 7.
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Correlations between variability in R-R intervals and feeling of stress in Task D (spot the difference).

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According to Spearman’s rank correlation analysis, the variability in median R-R intervals in Task A had strong positive correlations with feelings of stress; all of these correlation coefficients were greater than 0.7, with the highest coefficient being 0.840 (P < 0.001), as shown in Fig. 6. The means (± SD) of the differences in Fig. 6a and b were −58.1 (± 86.8) and −58.6 (± 87.8) ms, respectively. More specifically, participants did not appear to report a feeling of stress when their R-R intervals quickened by approximately 150 ms; this rate is the means minus SD. In this way, we can predict the strength of the feeling of stress according to heart rate variability when individuals engage in relatively creative tasks such as the “marshmallow challenge”.

In contrast, we observed no significant correlations between variability in median R-R intervals in Task D and feeling of stress; indeed, the highest correlation coefficient was no more than 0.3. The mean differences in Fig. 7a and b were approximately zero: −1.50 (± 48.0) and −0.595 (± 46.3) ms. It must be noted that, as shown in Fig. 8, the strength of the feeling of stress during attempts at Task D was comparable with that during attempts at Task A. However, the heart rate variability in the simple tasks did not significantly correlate with a feeling of stress; thus, it is difficult to estimate feelings of stress based on heart rate variability when the individuals are attempting simple tasks.

Fig. 8.
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Strength of feeling stress in each Task (spot the difference).

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Overall, feelings of stress are not likely to be predicted by median R-R intervals for Tasks C and D. However, it is possible that the conditions of the initial rests influenced the R-R intervals during each attempt, and therefore would influence the above-mentioned correlation analysis results.

4 Conclusion

The purpose of this study is to clarify how creative and non-creative work influences heart rate variability. We found that patterns of R-R intervals differed noticeably between creative and non-creative work, with the heart rate quickening during creative work and recovering during the subsequent rest. Additionally, the variability in median R-R intervals in creative work was strongly and positively related to feelings of stress (with most correlations being > 0.7 and the highest being 0.840; P < 0.001). These results suggest that it is possible to predict feelings of stress according to heart rate variability when people are engaged in creative work. In contrast, variability in median R-R intervals in non-creative work was not significantly correlated with feelings of stress (with the highest coefficient being no more than 0.3).