The human heart beats about 100,000 times per day. The time interval between each heartbeat changes from moment to moment. Heart rate variability (HRV) is a measure of these time fluctuations between heartbeats over time. When relaxed, our hearts show greater momentary beat-to-beat variation, and so greater HRV. This is due to the predominance of parasympathetic over sympathetic nervous system activity when we are not under stress. Researchers consider increases in HRV to be an objective measure of stress reduction.
Preliminary studies show mindfulness meditation increases HRV. However, these studies often rely on one-time measurements, leaving little known about the persistence of HRV changes over time. Kirk et al. [PLOS One] studied short- and longer-term changes in HRV before, during, and after an app-based mindfulness training compared to two control groups. The study is unique in obtaining HRV measures in participants’ home environments rather than in a laboratory setting.
The researchers randomly assigned 90 Danish participants (average age = 37 years; 70% female) to a 10-day mindfulness training using the Headspace app, a 10-day app-based music listening control condition, or a no-intervention control. The Headspace guided meditation sessions were 20-minutes in length on days 1-5, and 30-minutes in length on days 6-10. Mindfulness training included focused-attention on the breath and body, awareness of mind-wandering, and developing a non-judgmental attitude.
The music listening app consisted of a playlist of 20 two- to four-minute music tracks offered in 20- and 30-minute installments. The inactive control group conducted their lives as usual during the 10-day intervention period.
All participants had their HRVs continuously monitored using a wearable device while going about their normal daily activities for 48 hours prior to and 48 hours following the intervention period. In addition, participants in the mindfulness and music groups had their HRVs monitored whenever they were using the assigned intervention app, allowing researchers to monitor HRV over the 10 intervention days. They also completed self-report measures of perceived stress and mindfulness (the Mindful Attention Awareness Scale).
Results showed significantly larger increases in mindfulness (η2p=.14) and larger decreases in perceived stress (η2p=.08) after intervention for the mindfulness group over each control groups. The mindfulness group showed significantly greater increases in average daytime (η2p=.12) and nighttime (η2p=.30) HRV after intervention than each control group.
Increases in post-intervention daytime HRV (r=.47) and nighttime HRV (r=.44) were significantly correlated with total minutes of meditation practice during intervention. Music group nighttime increases in HRV were also correlated with time spent listening to music (r=.36).
There were also acute HRV effects that occurred while meditating or listening to music. The mindfulness group showed a significant 13 millisecond average increase in HRV over baseline while meditating. The music group showed a significant 9 millisecond increase over baseline while listening to music.
The mindfulness group showed a respiration rate that significantly decreased by four breaths a minute compared to baseline while meditating, while the music group did not slow respiration while listening to music.
The study shows that an app-delivered 10-day mindfulness intervention increases HRV while meditating, and that increases in HRV persist for 48 hours after the 10-day training period. Total minutes of meditation correlated with increased HRV. The study demonstrates the feasibility of assessing HRV as an objective biomarker in naturalistic environments outside the laboratory. The analysis in the study is limited by not adjusting for respiration rate and tidal volume which can directly impact HRV.
Kirk, U., & Axelsen, J. L. (2020). Heart rate variability is enhanced during mindfulness practice: A randomized controlled trial involving a 10-day online-based mindfulness intervention. PLOS ONE, 15(12), e0243488.[Link to study]