Controlled studies have demonstrated that light supplementation benefits circadian entrainment, sleep and mood. Translating these findings into effective, real-world usage of light supplementation is, however, challenging because naturalistic and supplemental light exposure are shaped by personal habits and environmental light conditions. Bridging this translation gap is especially critical for older adults, given the reduced ocular light sensitivity associated with natural ageing, which exacerbates low daytime indoor light exposure in urban housing. We therefore set out to investigate the determinants of self-directed light supplementation in this population, and what the real-world feasibility and efficacy of daytime light supplementation is. We recruited 202 participants (63-92 years; 139 females) to our interventional, prospective, randomised study in three major EU cities at different latitudes (Amsterdam, Bologna, Tartu). Participants underwent a 2-week baseline assessment of light exposure, rest-activity patterns and sleep through wearables and daily diaries. Participants were then randomised to either 12 weeks of self-implemented indoor light supplementation using a lamp placed in their most frequently used room (> 8000 melanopic EDI, N = 98) or a no-intervention group. A subsequent 2-week reassessment showed that greater naturalistic light exposure was associated with increased daytime activity, consolidated wakefulness, better subjective sleep quality, and reduced metabolic disorder incidence. Individuals with lower naturalistic light exposure self-initiated earlier and longer indoor light supplementation across seasons, in which supplementation onset before 10:00 was associated with greater daytime activity and lower rest-activity fragmentation. These results confirm that this simple, non-pharmacological, and low-cost intervention has broad potential to support healthy ageing in urban settings.
