Abstract This manuscript presents two studies on the effect of mindfulness meditation on duration judgment and its relationship to the subjective experience of time when the interval durations are on the second or the minute time scale. After the first 15 minutes of a 30-min meditation or control exercise, meditation-trained participants judged interval durations of 15 to 50 s or 2 to 6 min, during which they performed either a mindfulness meditation exercise or a control exercise. The participants’ scores on the self-reported scales indicated the effectiveness of the meditation exercise, as it increased the level of present-moment awareness and happiness and decreased that of anxiety. The results showed an underestimation of time for the short interval durations and an overestimation of time for the long intervals, although the participants always reported that time passed faster with meditation than with the control exercise. Further statistical analyses revealed that the focus on the present-moment significantly mediated the exercise effect on the time estimates for long durations. The inversion in time estimates between the two time scales is explained in terms of the different mechanisms underlying the judgment of short and long durations, i.e., the cognitive mechanisms of attention and memory, respectively.

Citation: Droit-Volet S, Chaulet M, Dutheil F, Dambrun M (2019) Mindfulness meditation, time judgment and time experience: Importance of the time scale considered (seconds or minutes). PLoS ONE 14(10): e0223567. https://doi.org/10.1371/journal.pone.0223567 Editor: Myrthe Faber, Radboudumc, NETHERLANDS Received: January 15, 2019; Accepted: September 24, 2019; Published: October 18, 2019 Copyright: © 2019 Droit-Volet et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability: All relevant data are within the paper and its supporting information file. Funding: The authors received no specific funding for this work. Competing interests: The authors have declared that no competing interests exist.

1 Introduction In recent years, there has been an explosion of interest in meditation. This enthusiasm for meditation results from the abundance of studies demonstrating its benefits for human beings. Meditation practice does indeed increase the feeling of well-being [1,2], reduces negative affects (anxiety, depression) [3,4,5,6], and enhances some attentional skills [7,8,9,10]. The practice of meditation also changes the relationship to time. However, the relationship between time and meditation remains a mystery, because it is largely under-investigated [11]. The practitioners of different meditative techniques report the feeling of being outside time when they meditate, as if time no longer exists [12,13]. This “timelessness” is often accompanied by the subjective experience of an acceleration of the passage of time during meditation, when the passage of time judgment (PoT judgment) is assessed with a self-reported scale indicating how quickly time seems to pass [14]. This feeling that time passes faster during meditation has been recorded in long-term meditators [15,16], and even in students practicing a mindfulness meditation exercise for the first time [17]. People who are more mindful also feel that time passes more quickly in a timing task [18]. The question that may be raised is: What does this self-reported feeling of the passage of time mean exactly? Is this PoT feeling linked to the judgment of durations? To try to answer these questions, recent studies have begun to experimentally examine the effects of meditation on the judgment of durations. Several studies have reported no difference in the judgment of short durations between long-term meditation practitioners and control subjects in a wide series of temporal tasks [19,20,21,22]. In addition, the few studies which have detected a difference have indicated better temporal judgments in meditation practitioners without any greater time distortions. Schötz et al. [23] found that experienced meditators were more accurate and precise in their time judgments. Wittmann et al. [24] observed higher temporal accuracy and precision in people who scored higher for trait-mindfulness on a personality scale. In the same way, Droit-Volet, Fanget and Dambrun [25] observed an increase in sensitivity to time after a mindfulness meditation exercise (see also [26]). Therefore, time distortions in duration judgment tasks, which might be expected on the basis of self-reported experience of the acceleration of the passage of time, are not observed in mindful people, in experienced meditators, or just after a mindfulness exercise. Some studies have therefore directly tested the judgment of durations during a mindfulness meditation exercise in which the state of consciousness is altered. Droit-Volet and Heros [22] administered short auditory stimuli (0.8, 1.2, 1.6, 2 s) during the last 15 minutes of a long mindfulness meditation session of 30 minutes. The results did not show any difference between the temporal judgments of experienced meditators and students with no previous exposure to meditation. All the participants underestimated the stimulus durations presented during the meditation exercise compared to those presented during a control exercise. Droit-Volet et al. [16] obtained the same underestimation of time during a meditation task compared to a control task when the participants did not have to judge stimulus durations but, instead, the temporal intervals (15, 30, 60 s) that had elapsed. This shortening of interval durations was observed with different mindfulness techniques (i.e. body scan, breathing meditation), and compared to different attentionally demanding control tasks. Despite a certain inter-individual variability, Glicksohn et al. [15] also observed a subjective shortening of stimulus durations (4, 8, 16, 32 s) when experienced meditators were placed in an altered sensory environment (i.e. whole body perceptual deprivation chamber), as was indicated by the longer durations that they produced in the temporal production task used. The results of studies indicating an underestimation of durations during a meditation exercise have logically been explained in terms of attentional processes [16,22]. According to attentional models of the internal clock [27,28], the subjective duration depends on the amount of attentional resources allocated to the timekeeper (clock). The smaller the amount of attentional resources allocated to time, the smaller the number of time units counted by the timekeeper is, and the shorter time is estimated to be. The predictions of attentional-clock models have been validated in a wide series of studies using a dual-task or attentional interference paradigm [29,30]. Therefore, the shortening of estimated duration observed during a meditation exercise should result from the fact that this specific exercise captures attention more than most attentional tasks do. However, the studies on the judgment of durations during a meditation exercise have examined only short durations (< 60 s), i.e. a time scale which does not correspond to the time experience reported by meditators. Although the temporal span considered by meditators when they report an acceleration of the passage of time is not clearly defined, it likely covers a period of time longer than 60 s, i.e., the entire period of the exercise, or at least several minutes. There are only two studies which have examined the effect of a meditation exercise on the judgment of such long temporal intervals, i.e., 13 minutes in Thönes and Wittmann’s [17] study and 5 minutes in Sucala and David’s study [31]. The first study observed that duration judgment was more accurate with a mindfulness meditation exercise (body-scan) than with a control exercise (relaxing to music), while the second study found no meditation effect on duration judgments. In addition, the passage of time was judged to be faster in the first study and slower in the second one. The inconsistency in the results of these two studies may be due to the use of different methods. In addition, they used a retrospective time judgment task which was different from the prospective time judgment task used in the studies reported above. Unlike in the prospective time judgment task, the participants in the retrospective time judgment task are not informed that they will have to judge time. The aim of this present study was thus to test the effect of a meditation exercise on the prospective time judgment of long interval durations of several minutes compared to that of shorter interval durations. Only a few studies have examined the judgment of long durations of several minutes. It has nevertheless been suggested that the mechanisms involved in the judgment of long durations are different from those involved in the judgment of short durations [32,33]. The judgment of long durations would be largely based on memory processes, similar to those observed in the retrospective judgment of durations, and the judgment of short durations on the functioning of a timekeeper (internal clock system) that demands attentional resources. According to memory-based models of the retrospective judgment of durations [34,35,36], time estimates are a function of the amount of non-temporal information stored and retrieved in memory, namely the characteristics of the experiencer (emotion), the events (number, complexity) or the activity (effortless, attentionally demanding) performed during the time period [36,37]. The more attentionally demanding the activity performed during the temporal interval is, the longer the elapsed duration is retrospectively judged to be [36,37]. Consequently, whether the meditation exercise is an attentionally demanding task, we can assume that the practice of a meditation exercise should result in a temporal underestimation for short interval durations and a temporal overestimation for long interval durations of several minutes. In addition, some recent studies have indicated that awareness of the passage of time (PoT judgment) and duration judgment are dissociated on short time scales, but linked to each other on long time scales of several minutes. Droit-Volet and her collaborators found that the awareness of the speed of the passage of time was a significant predictor of duration judgments for long intervals of several minutes [33,38,39]. In addition, the best predictors of the PoT judgment were the emotion and the activity—difficult to achieve or requiring attention—experienced by the participants during the long temporal interval to be estimated. The different links between the awareness of the speed of the passage of time and the duration judgment for different temporal scales reinforce the idea that different mechanisms underlie the judgment of short and long durations. In the present study, we therefore examined the effect of a meditation exercise on both the judgment of durations and the judgment of the passage of time for both short and long interval durations. In our study, the participants, who had been trained in the practice of meditation, therefore had to judge interval durations belonging in two duration ranges, that of seconds and that of minutes, during either a mindfulness meditation exercise or a control exercise. The judgment of the passage of time during these exercises was also assessed. As in the procedure used by Droit-Volet et al. [16], the participants performed a temporal task during the last 15 minutes of a long 30-minute exercise. In addition, several studies have shown that the most important criteria determining the effectiveness of a meditation exercise are its effects on present awareness, anxiety and happiness. Indeed, the practice of meditation increases the awareness of the present moment and the feeling of happiness and decreases the anxiety level [2,6,40,41]. In our study, we thus also assessed these three psychological dimensions on self-reported scales. The participants filled in the scales before and after the meditation/control exercise as well as before the training phase, since meditation training can modify these dimensions. Our hypothesis was therefore that performing a meditation exercise should produce distortions in interval duration judgments compared to performing a control exercise. However, this distortion should take the form of a temporal underestimation for short durations in the seconds range, and a temporal overestimation for longer durations in the minutes range. In addition, we can assume that there will be a significant relationship between the feeling that time passes faster during meditation and the judgment of durations for the temporal intervals of several minutes, but not for those of a few seconds.

4 General discussion Experiments 1 and 2 showed a reduction in self-reported measures of negative affects (anxiety, sadness). This affective improvement appeared immediately after a meditation exercise, but also in the longer term as indicated by the significant effect of our daily-mindfulness training. These findings are consistent with the outcomes reported in the mindfulness training literature showing that the reduction of negative affects (fear, anxiety, sadness, depressive symptoms) is one of the main effects of mindfulness training [48]. The long-term benefits might be higher with 8–10 weeks of training than after a short period of training (7 days) such as that used in our study. A recent meta-analysis nevertheless testifies to the significant effect of brief mindfulness interventions on affects [49]. In addition, the length of a brief daily home training program did not moderate, or moderated only slightly, the effect on affects [49]. As suggested by Nair et al.’s study [51], it is possible to move rapidly into a reliable state of meditation after only a short period of meditation practice. Our studies also suggested that the immediate positive effects of a mindfulness exercise were related to the regulation of emotion (decrease in anxiety and arousal level, increase in happiness), as well as to the activation of attention control mechanisms [52,53,54]. The participants indeed reported that the mindfulness exercise placed them in the present-moment, was attentionally demanding and imposed an information processing load due to its difficulty. This is consistent with studies showing that mindfulness exercises facilitate attentional focus and reduce distracting thoughts such as mind-wandering and rumination [55,56,57]. However, the true originality of our studies lies in examining the effects of a mindfulness exercise and its underlying processes (attention control, emotion) on different types of time judgments, both the judgment of interval durations and that of the passage of time. Our results showed that the interval durations of several seconds (from 16 to 50 sec) were underestimated with a mindfulness exercise compared to a control exercise. As already reported, these results replicated those found in a similar task of interval duration judgment with two different types of mindfulness techniques (body scan, breathing) and different activities used as a control task [16]. They also replicated those found in a task of stimulus duration judgment in both experienced meditators and participants with no prior meditation experience [22]. The shortening of durations in the seconds range in a mindfulness task is therefore a robust result. As stated in the Introduction, this shortening of time has been explained in the framework of predictions of attention-clock models that have been widely empirically validated [27,28]. According to these models, the judgment of durations directly depends on the amount of attentional resources allocated to time processing. It is thus likely that the shortening of short durations observed in our study with the meditation exercise results from the fact that the attentional focus is directed on the meditation activity at the expense of time processing. The smaller the quantity of attentional resources allocated to time processing, the shorter the perceived duration is. Obviously, a mindfulness exercise cannot be reduced to an attention task, as its effects on affects suggest. It is nevertheless an attention control task that consumes cognitive resources, as evidence by the statements of our participants in Experiment 2. In addition, numerous studies have demonstrated that regular practice of this type of mindfulness exercise increases attention skills because people train their attention control activity [58]. However, the temporal shortening observed for the estimation of short interval durations was not observed for that of longer interval durations of several minutes. Instead, the direction of the time distortion was reversed, i.e. a temporal lengthening instead of a temporal shortening. Indeed, in our study, the interval durations of several minutes were clearly overestimated in the meditation exercise compared to the control exercise. In addition, this temporal overestimation was observed in Experiment 1 and replicated in Experiment 2, attesting to the consistency of this result. To better understand this lengthening of time at the time scale of minutes, we questioned the participants on their experience, during the temporal interval that had elapsed, in terms of attention, task difficulty, focus on the present and arousal level. The statistical analyses revealed that the temporal lengthening of long durations produced by the meditation exercise compared to the control exercise was significantly linked to self-reported measures of attention (attention, difficulty) and focus on the present-moment. The more attentionally demanding and requiring a focus on the present the meditation exercise was judged to be, the longer the time estimates were. However, the mediation analysis revealed that only the self-reported assessment of being focused on the present moment significantly mediated the relationship between the meditation exercise and the time estimates. However, present-moment awareness and the attentional demand of the exercise were closely related. As indicated by our results, the scores on the attentional-demand scales were significant predictors of scores on the present-moment awareness scale. This suggests that time estimates at the temporal scale of minutes depend on the attentional focus on the present moment induced by the exercise. In the memory-based models, similar time extensions observed in retrospective time judgments are explained in terms of the size of memory storage [32,35,36]. The idea is that the retrospectively judged duration is a function of the amount of information stored during the temporal interval (amount of information, information complexity). Since then, it has been suggested that the segmentation of activity or the number of perceived changes also play a role [36]. As explained by Block [37], there is a long list of contextual factors that may influence the retrospective time judgment. Whatever these factors, the retrospectively judged duration depends on the non-temporal content of the interval to be timed, such as the workload involved in processing non-temporal information: the higher it is, the longer time is considered to be. Consequently, we may assume that the mindfulness meditation exercise had the same attentional impact on the participants in all conditions, but that this led to opposite results depending on the temporal scale, because the processing of long durations is largely dependent on a cognitive memory mechanism similar to those observed in the retrospective time judgment, and the processing of short duration is dependent on an internal clock system, whose operation requires attention. This provides additional support for the idea that the processing of short and that of long durations do not share the same mechanisms [39]. Another unsolved question is that of the temporal point at which we move from one mechanism to another, or rather the point when the high-level cognitive processes take over from the lower-level processes, because a clock system and a non-temporal processor can operate in parallel [59]. Our study does not allow us to answer this question, but it suggests that the awareness of passage of time is related to present-moment awareness and contributes to duration judgment during high-level cognitive processing. In Experiment 2, when the PoT judgment was assessed for each interval to be timed, it appeared to be a significant predictor of estimated durations in the range of minutes, and vice versa. The PoT judgment and the duration judgment are thus significantly linked for the judgment of long durations of several minutes. This is consistent with the studies using ecological momentary assessment methods that have shown that the PoT judgment is a significant predictor of long durations, but not of short durations going from a few milliseconds to several seconds [38,39]. Consequently, time awareness plays a critical role in the different forms of time judgments for long intervals of several minutes. In addition, in Experiment 2, for each interval duration, the participants had to describe their experiences while they were practicing the exercise requested. They stated that, in comparison to the control exercise, they were calmer during the meditation exercise, more focused on the present-moment, and that their attention was focused on this task, which they judged more difficult. The statistical analyses revealed that time was judged to pass faster when the participants felt calmer and when their attention was focused on the exercise and the present moment, the two being obviously linked. Therefore, the more attention was focused on the required exercise, the longer the interval dedicated to this exercise was considered to be, and the faster external time was judged to pass. However, the mediation analyses did not show any significant indirect effect of these different dimensions (attention, difficulty, present) on the relationship between the meditation exercise and the PoT judgment. Nevertheless, the self-reported level of attention allocated to the exercise significantly contributed to the total effect of the meditation exercise on the PoT judgment when it was included in the statistical model. Therefore, the subjective experience of an acceleration of time arises partially from the fact that the cognitive resources are fully occupied by the realization of the mindfulness activity. However, the effect of mediation on the feeling that time passes faster cannot be reduced to attentional effects. Other dimensions must be examined, such as the sense of self and of body [13]. Most meditators say that their sense of time is altered when they meditate because they are outside time, i.e. in a state of timelessness. Our studies provide empirical data suggesting that this phenomenological description of their relationship to time arises to a large extent from their introspective analysis of their own internal state (attentional focus on the present activity) during the meditation experience. This conscious analysis of their internal state is thus translated into a feeling of duration—“self-duration”- compared to the representation of “world-duration” (external time) [13,60]. This allows them to state that time goes faster, because time is outside their mind, their attention being focused on the present-moment. However, further investigations are now required in experienced meditators because a meditation exercise may produce for them a state different from that observed in participants who have received only a short period of training, although the results of several studies do not support this idea [19,20,21]. Other investigations are also required to test durations longer than those used in our study (> 6 min), because the temporal task administered during the exercise may perhaps have interfered with meditation performance, although the interference effects were reduced with the long interval durations of between 2 and 6 minutes used in our experiments. It is also important to try to better understand the links in our study between the focus on the present-moment, attention and task difficulty in the experiences reported by the participants. The feeling of being focused on the present-moment may go beyond a problem of attentional focus. Other investigations are indeed needed to examine other dimensions, such as the sense of self which also play an important role in the mindfulness experience, especially in experienced meditators. It is in fact difficult to identify the influence of one major dimension, as the general state of consciousness is altered by mindfulness meditation.

Supporting information S1 File. Tables of data used for Figures and statistical analyses. https://doi.org/10.1371/journal.pone.0223567.s001 (XLSX)