Non-invasive close to real-time assessment of dynamic pupil responses to different light stimuli and the relationship between those pupil responses and circadian rhythm and sleep metrics

The National Heart, Lung, and Blood Institute awarded The General Hospital Corporation (Massachusetts General Hospital) $159,560 on December 1, 2026, under the Cardiovascular Diseases Research program (CFDA 93.837) to conduct non-invasive assessment of dynamic pupil responses to light stimuli and their relationship to circadian rhythm and sleep metrics.

The project tests the hypothesis that differences in pupillary response metrics to varying wavelength, duration, and intensity of light stimuli will correlate with circadian rhythm metrics and serve as a biomarker for individual differences. The work addresses the clinical need for accessible, real-time assessment tools to measure circadian rhythm alignment, which currently hinders translation of circadian science into clinical practice. The research focuses on post-illumination pupillary response (PIPR), a response mediated by intrinsically photosensitive retinal ganglion cells containing melanopsin, and examines whether variations in this response correlate with circadian rhythm sleep-wake disorders and other circadian misalignment conditions.

Performance occurs in Boston, Massachusetts. The period of performance runs from December 1, 2026, through April 30, 2027. This is a project grant, an assistance type supporting investigator-initiated research training and competitive grants within NHLBI's portfolio of heart, lung, and blood disease research.

Abstract

The lack of non-invasive, real-time assessments of circadian rhythm metrics hinders the translation of circadian rhythm knowledge into clinical practice. Circadian rhythms are internal physiological processes that cycle every ~24 hours. Light is the most potent stimulus for circadian rhythm entrainment to environmental time. Misalignment of the circadian system with environmental time increases the risk of both chronic diseases (e.g., cardiovascular disease, obesity) and acute mental and physical health impairments, underscoring the need for accessible clinical assessment tools. The mechanisms underlying individual variability in circadian (MIS)alignment are unclear; one potential mechanism would involve variations in how light is processed in the eye. This project will test the hypothesis that differences in multiple metrics of pupillary response to light stimuli will correlate with differences in circadian metrics - and be a potential biomarker for individual differences. Wavelength, duration, and intensity of light differentially affect the response of retinal image-forming (e.g., rods, cones) cells and non-image-forming (NIF) intrinsically photosensitive retinal ganglion cells (IPRGCS) that contain melanopsin. The NIF pathway is crucial for circadian entrainment to environmental time and impacts the pupillary light reflex, melatonin concentrations, and other physiology. Variations in the post-illumination pupillary response (PIPR), an iprgc- linked response, have been linked to neurological conditions, including circadian rhythm sleep-wake disorders (CRSWD), suggesting that pupillary response could be used as a potential non-invasive close-to-real-time diagnostic and monitoring tool. Relationships between pupil responses during and after light stimuli and circadian timing metrics should be established. Our specific aims are to quantify (I) different metrics of pupil response during and after light stimuli of various intensities, wavelengths, and durations (II) and their relationships with DIM-light melatonin onset (dlmo, the current gold standard measurement for circadian phase) (aim 1) and in individuals with two intrinsic circadian sleep-wake phase disorders (CSWPD) with altered sleep timing (aim 2). An outpatient week of monitoring of sleep timing will be immediately followed by the inpatient protocol for both participant groups (aim 1, healthy controls, leveraging a funded R01 experiment; aim 2: individuals with CRSWD funded with the F32 funds). The inpatient protocol will consist of multiple pupillometry sessions and an evening saliva collection for dlmo. During pupillometry, participants will be exposed to wavelengths of light designed to target the iprgc cells (blue light) or not (red light) with a matched number of photons of different intensities and durations. This F32 award will incorporate training in the measurement and analysis of physiological measures, circadian rhythms, and photobiology, phenotyping of people with CSWPD, advanced statistical training, grant and manuscript writing, scientific communication, and lab management. Establishing a strong foundation in these areas will facilitate DR. Mccullar's transition to an independent scientist uniquely poised to establish outpatient testing that provides non-invasive close-to-real- time assessments of circadian metrics that can be used to evaluate and monitor individuals with circadian (or other NIF- linked) pathologies, as well as tools to help facilitate the integration of circadian system monitoring into clinical practice.