This study aims to evaluate how different heart rate variability (HRV) recording durations reflect autonomic nervous system responses during transcutaneous auricular vagus nerve stimulation (taVNS) in healthy adults. In a randomized, sham-controlled crossover design, participants will attend two separate sessions and receive both active taVNS and sham stimulation in randomized order. During each session, a continuous 10-minute HRV recording will be obtained, and 1-minute, 5-minute, and 10-minute analysis windows will be compared. Blood pressure and pulse will also be measured before and after stimulation. The study will investigate whether shorter or longer HRV recording durations provide results comparable to the standard 5-minute recording and which duration most reliably detects autonomic changes during taVNS.

The vagus nerve plays a central role in autonomic regulation, and heart rate variability (HRV) is a widely used noninvasive marker of autonomic nervous system activity. Transcutaneous auricular vagus nerve stimulation (taVNS) has increasingly been used in experimental and clinical research as a noninvasive method to modulate vagal activity. Although previous studies suggest that taVNS may influence parasympathetic activity, HRV measurement duration remains an important methodological issue. Standard short-term HRV analysis is commonly based on 5-minute recordings, but ultra-short and longer recordings are also used, and their validity during taVNS has not been sufficiently clarified.

This study is designed to determine whether different HRV recording durations provide comparable and reliable information during taVNS. The study will use a randomized, sham-controlled crossover design. Healthy adult participants will attend two sessions separated by a washout period of 48 to 72 hours. In one session, participants will receive active taVNS; in the other, they will receive sham stimulation. The order of sessions will be randomized.

During each session, HRV data will be collected continuously for 10 minutes using a Polar H10 chest strap. From this single uninterrupted recording, 1-minute, 5-minute, and 10-minute analysis windows will be derived. HRV parameters from time-domain and frequency-domain analyses will be calculated. Blood pressure and pulse will be measured before and after the intervention using an automated upper-arm blood pressure monitor.

Active taVNS will be applied to the cymba conchae region of the ear using commonly reported stimulation parameters. Sham stimulation will be delivered with the same device and similar sensory conditions, but at a non-vagal ear location to minimize physiologic vagal effects. This design is intended to control for nonspecific sensory and expectation-related effects.

The primary objective is to assess whether the standard 5-minute HRV recording during taVNS detects autonomic effects compared with sham stimulation. Secondary objectives are to examine the agreement between 1-minute, 5-minute, and 10-minute HRV measurements and to evaluate whether shorter or longer recordings provide additional methodological value. The relationship between HRV findings and hemodynamic responses, including blood pressure and pulse, will also be explored. This study may help identify the most appropriate HRV recording duration for future taVNS research and contribute to methodological standardization in this field.

Inclusion Criteria:

• Healthy volunteers aged 18 to 40 years
• Able and willing to provide informed consent
• Able to tolerate active transcutaneous auricular vagus nerve stimulation and sham stimulation
• Able to comply with study instructions, including restrictions related to caffeine, nicotine, and heavy exercise before assessment
• No condition preventing heart rate variability, blood pressure, or pulse measurements

Exclusion Criteria:

• Diagnosed cardiovascular disease, arrhythmia, or pacemaker use
• History of neurologic disease that may affect autonomic nervous system function
• Regular use of medications that may affect heart rate variability or autonomic responses, including beta blockers, antiarrhythmics, or anticholinergic drugs
• Active ear infection, dermatologic lesion, or any ear condition that may interfere with stimulation
• Marked pain, discomfort, or intolerance during stimulation
• Acute infection, fever, or other acute health problem on the measurement day
• Heart rate variability data that cannot be evaluated because of excessive motion artifact

The study will use partial blinding. The investigator performing physiologic measurements and the researcher responsible for statistical analysis will be blinded to intervention assignment. Data will be analyzed using coded datasets, and group identities will be revealed only after completion of the analyses. The researcher delivering the intervention cannot be blinded because active taVNS and sham stimulation require different application sites.