Effects of bright light and near-infrared light on insomnia symptoms in community-dwelling older adults: a randomized clinical trial

BACKGROUND: Sleep disturbances are highly prevalent among older adults and associated with substantial morbidity. Although bright light therapy has shown efficacy for insomnia, the evidence supporting near-infrared (NIR) light is comparatively limited. AIMS: To examine the effects of NIR (850 nm photobiomodulation to the neck), white light (WL, to the eyes), and their combination on sleep and melatonin secretion in older adults with insomnia symptoms, and explore whether chronotype moderated treatment outcomes. METHODS: A randomized clinical trial was conducted among 59 community-dwelling adults aged ≥ 60 years with early wakening and sleep-maintenance insomnia symptoms. Participants were randomized to NIR, WL, or NIR+WL for 60 min nightly over two weeks. Data gathered included actigraphy-derived sleep parameters, Pittsburgh Sleep Quality Index (PSQI), salivary melatonin concentration, and dim-light melatonin onset (DLMO). Chronotype was assessed with the Munich Chronotype Questionnaire, and its moderating effect was examined. RESULTS: PSQI scores improved significantly across all intervention groups, with increases in subjective sleep duration of 0.43 h (WL), 0.81 h (NIR), and 1.08 h (NIR+WL). No significant changes were observed in actigraphy-derived sleep indicators or melatonin measures. Chronotype significantly moderated treatment effects, with late chronotypes demonstrating greater improvement in sleep duration and efficiency with WL, whereas early chronotypes showed greater improvement with NIR+WL. CONCLUSIONS: Both NIR and WL were associated with improvements in subjective sleep quality and duration. Chronotype-specific patterns of response suggest distinct underlying mechanisms and support the potential for personalized light-based interventions for insomnia in older adults. CLINICAL TRIAL REGISTRATION: This study was registered on ClinicalTrials.gov (https://clinicaltrials.gov/study/NCT06292819).