Photobiomodulation (PBM) is a form of light therapy that uses red or near-infrared light to influence biological processes. In prior studies, PBM has been shown to improve various physiological functions, including tissue repair, inflammation, and cognitive performance. However, little is known about its impact on autonomic nervous system regulation. Thus, the goal of this study is to evaluate the effects of PBM on vagal activity, as measured by heart rate variability (HRV), in healthy adults undergoing experimental stress.

Photobiomodulation (PBM) is a non-invasive therapeutic technique that uses low-level light, typically in the red or near-infrared spectrum, to stimulate biological tissues. This approach has been increasingly studied for its beneficial effects on wound healing, mood, and sleep quality. Recent research has also highlighted PBM's potential in enhancing brain function, neuroplasticity, and cognitive performance. For example, prior studies have demonstrated that transcranial PBM (i.e., PBM delivered to the head) can help modulate neuronal activity, improve cerebral blood flow, and promote neuroprotective effects for traumatic brain injury and stroke. Given these findings, PBM may also have potential for influencing autonomic function and stress physiology. However, to date, little is known about the effects of PBM on the autonomic nervous system, particularly its influence on vagal activity and heart rate variability (HRV).

This study aims to fill this gap by investigating the effects of transcutaneous cervical plus intranasal PBM on HRV in healthy individuals subjected to experimental stress. Specifically, the Maastricht Acute Stress Test (MAST) will be used, which is a validated stress protocol that has been shown to strongly and reliably induce acute autonomic stress responses. A within-subject design will be employed with four repeated visits per participant to examine the effects of both PBM and sham intervention applied before and after stress induction on vagal activity, as measured by HRV. As the first study to examine PBM's impact on HRV, this research will provide valuable insights into the potential of PBM as a novel, non-invasive approach to modulating autonomic regulation, particularly in the context of stress.

Inclusion Criteria:

• Healthy adults, aged 18-64 years
• Ability to read and understand English
• Willing to comply with the study procedures and attend all scheduled sessions

Exclusion Criteria:

• Taking medications that could significantly influence autonomic function
• Presence or history of any serious cardiovascular, inflammatory, and pain conditions as well as neurological disorders or other conditions that affect the central nervous system
• Pregnant