A Digital Mindfulness Intervention to Improve Sleep Efficiency and Heart Rate Variability in Healthy Adults

This randomized controlled trial (RCT) used a waitlist-controlled design to examine whether a 10-day smartphone app-based mindfulness program could improve sleep and reduce physiological stress markers in healthy adults. The study was embedded within a broader 4-month longitudinal cohort study (N=140); the RCT sub-study focused on participants in the low-stress stratum (Perceived Stress Scale [PSS-10] ≤13) at screening. Participants (n=81) were randomized in approximately a 3:2 ratio to either a mindfulness intervention group (n=49) or a waitlist control group (n=32). The mindfulness group completed one 10-minute guided audio session daily ("destress" module, narrated by Headspace) delivered via the Ōura app for 10 consecutive days in free-living conditions at a time of their own choosing. Compliance was monitored objectively via app-recorded session timestamps. The waitlist control group wore the Ōura Ring and completed all questionnaires identically without receiving any intervention. All participants wore an Ōura Ring Gen 3 throughout the study to objectively and continuously measure sleep architecture, sleep efficiency, and heart rate variability (HRV). Self-report questionnaires on perceived stress (Perceived Stress Scale; PSS-10), sleep quality (Pittsburgh Sleep Quality Index; PSQI), burnout (Copenhagen Burnout Inventory; CBI), and mindful attention (Mindful Attention Awareness Scale; MAAS) were administered before, immediately after, and four weeks after the 10-day intervention period. The study was conducted at the University of Southern Denmark, Odense, Denmark.

Inclusion criteria

• Adults aged 20-60 years
• Body mass index (BMI) in normal range (18.5-29.9 kg/m²)
• Low stress at screening (Perceived Stress Scale (PSS-10) score ≤13)
• Willingness to wear the Ōura Ring continuously throughout the study
• No current regular mindfulness practice (defined as >20 min/week)
• Ability to provide written informed consent
• Sufficient Danish or English language proficiency

Exclusion criteria

• Current regular mindfulness or meditation practice >20 min/week
• Self-reported diagnosis of cardiovascular or neurodegenerative disease
• Current diagnosis of anxiety or depressive disorder
• Use of psychotropic medications, hypnotics, or medications affecting heart rate variability (HRV) or sleep
• Pregnancy
• Shift work or highly irregular sleep schedules
• Concurrent participation in another mindfulness or behavioral sleep intervention