The study was conducted at a Wellness Center where the clients come to address quality of life challenges such as poor sleep, anxiety, relationship issue, etc. Each subject signed a written consent and confidential disclosure agreement. The ethics committee approval was not needed due to the non-invasive nature of the intervention consistent with an earlier study 12.

A room with good soundproof interiors was used while maintaining 25 Degree Centigrade temperature. This ensured the minimum impact of outside sound and a pleasant indoor temperature. The exclusion criteria included the presence of (a) chronic disease (b) epilepsy (c) pacemaker in the body and (d) metal parts due to any surgery inside the body or (e) inability for the subject to lie down on the floor in supine position. The demographics of subjects in both groups are captured in Table 1. Out of 21 individuals who participated in Himalayan Singing Bowls session, data of 5 individuals were rejected due to either artifact quality or disturbance in recording. For Supine silence, only 3 individuals data was rejected out of 20. Table 1.

For Himalayan Singing Bowls sessions, the subjects were asked to lie down with eyes closed for 20 minutes. 7 Himalayan Singing Bowls (Full Moon Singing Bowls, Nepal) and a Tingsha were used (as shown in Figure 1). The bowls diameter ranged from 18cm (for the smallest bowl) to 29.5 cm (for the largest bowl). The bowls were hit with a mallet (resembles the hammer with a soft, puffy end that is used for hitting) and the gap between the successive hits was minimum of 5 seconds. The sequence started with the bowl between the legs (furthest away from the head) and slowly moved towards the bowl nearer to the head while alternating on either side of the body. The sequence began with medium (about 50% of the maximum) intensity sound and each subsequent cycle of 7 hits of bowls and 1 hit of Tingsha reduced the intensity marginally (e.g. 50%, 40% and so on). Once the intensity was the lowest (10%) the subsequent cycles increased the intensity progressively towards 50% and this sequence was continued till 20 minutes were completed. Each cycle was followed by a sound of Tingsha. The subject remained supine with eyes closed throughout the practice, though the eyes were not covered during both the types of sessions. No guided instructions were used throughout the 20 minutes. This method is part of a more comprehensive method taught by Master Shree Krishna Shahi (Nepal) to author Gunjan Y Trivedi in year 2016.

For Supine Silence, the exact same process was used (duration, position) and this was done in complete silence without any sound of singing bowls.

The results of key parameters for HSB and SS groups are captured in Figure 3, Table 2 and Table 3 respectively.

Average Stress Index continued to decrease in the HSB group, however for the SS group the reduction was not consistent, i.e. during the last 5 minutes (Table 4 vs Table 3 in Table 3), there was an increase.

RMSSD and HR showed a consistent trend in HSB (decrease in HR every subsequent 5 minutes and increase in RMSSD also during every subsequent 5 minutes). For SS, during the last 5 minutes the RMSSD decreased.

To understand if there were statistically significant changes in HSB group for Stress Index, HR and RMSSD as compared to the SS group, the next section analyzes the data statistically.

(Table 3 and Table 4) capture the Paired-T test results of SS and HSB groups respectively. The main objective was to confirm if the reduction in Stress Index is more significant in the HSB group as compared to the SS group. The secondary objective was to confirm if HR reduction and RMSSD increase is more consistent and significant in HSB as compared to SS group to validate the impact on the parasympathetic nervous system.

The analysis explored (a) how the parameters changed when each subsequent interval is compared with the first (starting) interval and (b) how parameters changed in each subsequent interval compared to the previous interval.

When interval T4 is compared vs T1, both HSB and SS show statistically significant reduction (Table 3A and Table 4A). However, when we probe further by comparing each session with the previous session (i.e. T2 vs T1, T3 vs T2, T4 vs T3 in Table 3B and Table 4B), the stress index reduction is still statistically significant for each 5 minutes duration in HSB group (p<.05) and this is not the case for the SS group. This confirms the primary objective that HSB can significantly reduce stress as compared to SS throughout the short session of 20 minutes.

Similar analysis for RMSSD and HR confirms that RMSSD reduction is not statistically significant when T4 is compared to T1 (Table 3A) for SS while HR reduction is statistically significant for T4 vs T1 in SS. In the HSB group (Table 4A), interval T4 vs T1 show a statistically significant increase in RMSSD and a statistically significant decrease in HR (p <.05). Further analysis (Table 3B and Table 4B) confirms that when each interval is compared with the previous interval, 2 out of 3 intervals show statistically significant changes in RMSSD and HR for HSB session whereas for SS, the RMSSD increase is not statistically significant in any of the intervals and HR decreases only in 1 out of the 3 intervals (p<.05).

To summarize, in the HSB group, there was a statistically significant stress index reduction as compared to SS during each subsequent 5 minutes interval confirming the primary objective. The reduction in HR and increase in RMSSD, in the HSB group is also more consistent as compared to SS. These changes indicate an enhancement of parasympathetic activity in the HSB group as compared to SS providing a confirmation of the depth of relaxation. Together, these results confirm the hypothesis that 20 minutes long supine session with HSB enables significantly more relaxation as compared to SS.

This study highlighted that key HRV parameters (including Stress Index) show an improvement indicating good relaxation and activation of the parasympathetic nervous system. However, a more detailed statistical analysis provided new insights into the depth and quality of relaxation. When each group data was compared by breaking down the 20 minutes of each session into 4 intervals (T1 to T4), for HSB group, the final interval (when compared to the first interval), showed a statistically significant change in all the 3 parameters i.e. stress index, RMSSD, and Heart rate. However, for SS group, only 2 of the 3 parameters showed statistically significant reduction (i.e. RMSSD did not decrease in a statistically significant manner in T4 as compared to T1).

Based on this data, another analysis was conducted which compared each 5 minutes interval with the previous 5 minutes interval to understand if there are variations for both the methods. The analysis revealed that for the HSB group, 7 out of 9 intervals across the 3 measures provided statistically significant changes in the parameters (p-value <0.05). For 2 out of the 9 intervals, the p-value was 0.06 (which is not statistically significant however it is not very far from the cut-off compared to the data for SS). This denotes a very consistent reduction, session by session, for 16 participants during the Himalayan Singing Bowls session that lasted only 20 minutes! This was not the case for SS which provided statistically insignificant changes in various parameters when analyzed session by session (only 3 out of the 9 sessions across the total 3 parameters showed statistically significant reduction, p-value<0.05)

This finding is significant since the total duration of the session was only 20 minutes and despite that, the Himalayan Singing Bowls, as compared to Supine Silence, were able to achieve consistent enhancement in parasympathetic nervous system parameters. Consistent reduction in such a short duration is very encouraging as compared to earlier experiments where the session duration was longer (e.g. 1 hour).If we consider the p-value, for Himalayan Singing Bowls sessions the p-value >0.05 happened for only 2 measures and in those cases, the p-value was 0.06 (not very far from 0.05. In comparison, for Supine Silence, for most sessions with not significant reductions, the p-value was higher (usually > 0.2).

Future work in this area could explore (a) more sample size (b) understand this by demographics such as age, gender and also compare individuals who have insomnia versus individuals who usually sleep well and (c) explore additional methods such as Guided Imagery, Hypnosis script, Yoga Nidra with silence and singing bowls. The objective is to achieve deep relaxation in a relatively short amount of time - to calm the mind and the body and achieve the benefits similar to what is reported by other supine practices such as Relaxation Response or Yoga Nidra . The findings from this study that only a 20 minutes long session with HSB can help in reducing stress and enhancing parasympathetic activity is significant and must be leveraged by health care professional. Some ideas for re-applications in health care centers are captured under Recommendations .