Débora Barroggi Constantino

Introduction: Light is more than a means for vision; it synchronises the body’s internal clock, influences alertness, mood, and broader physiology. With age, structural changes in the eye reduce retinal light input, and low indoor/outdoor light exposure aggravates this, contributing to the high prevalence of sleep problems in older adults. Although laboratory studies positively support light supplementation, real-world implementation is challenging, as light use is shaped by personal habits and environmental/social factors. Moreover, most research has examined circadian rhythms using isolated metrics at a single timescale, overlooking the full complexity of biological rhythm organisation in ageing. Aims: To investigate the effectiveness of real-world light supplementation on daily rhythms and sleep of older adults living at home, identify what determines engagement and response, and describe how biological rhythms are organised across multiple timescales and phenotypes with age. Methods and Results: Four studies were conducted. A crossover study (N=36) showed that guided morning blue-enriched light improved rhythm stability and reduced sleep fragmentation, whereas evening light worsened sleep latency and efficiency. A larger, multi-site, randomised study (N=202) of self-directed supplementation confirmed that earlier light use improved daytime activity and rest-activity consolidation, and that lower daylight exposure, later chronotype, and winter predicted greater compliance. Wavelet analysis of actigraphy data (N=173) revealed that as circadian amplitude declines with age, ultradian rhythms (30–390 min) became more prominent, particularly in those with less robust circadian organisation. Finally, cluster analysis (N=183) identified four circadian-light phenotypes (chrono-states) differing in daily routines, sex distribution and metabolic health. Conclusions: Together, these studies show that naturalistic and supplemental light are critical for circadian consolidation in ageing, that ageing reorganises biological rhythms across timescales, and that circadian organisation is multidimensional. This work supports integrating light exposure and chronobiology into personalised, evidence-based and scalable strategies for healthy ageing.