The bright and dark side of blue-enriched light on sleep and activity in older adults.

Low indoor light in urban housing can disrupt health and wellbeing, especially in older adults who experience reduced light sensitivity and sleep/circadian disruptions with natural aging. While controlled studies suggest that enhancing indoor lighting may alleviate the negative effects of reduced light sensitivity, evidence for this to be effective in the real world is lacking. This study investigates the effects of two light conditions on actigraphic rest-activity rhythms and subjective sleep in healthy older adults (≥ 60 years) living at home. Two photon-matched lights were compared; a control white light (4000 K) and a blue-enriched white light (17000 K) at two different intensities (300-450 lx and 1100-1200 lx respectively). Participants (n = 36, 25 female) completed an 11-week randomized, cross-over study, comprising 1 week of baseline, 3 weeks of self-administered light exposure (2 h in the morning and 2 h in the evening), and 2 weeks of washout for each light condition. Participants completed sleep diaries, wore a wrist actigraph and a light sensor necklace, and collected urine to measure 6-sulphatoxymelatonin. Longer duration of morning blue-enriched light significantly improved rest-activity rhythm stability and decreased sleep fragmentation. More time spent above 2500 lx increased actigraphy amplitude, daytime activity, and advanced bedtime. Evening light exposure, however, increased sleep latency and lowered sleep efficiency. Our findings show morning blue-enriched light is beneficial whereas evening light should be avoided. Optimal timing of self-administered light interventions thus may offer a promising strategy to improve sleep and rest-activity rhythms in older adults in real-world settings.