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.

The aim of the present study was to investigate the actigraphic sleep-wake and light exposure patterns of a 48-y-old healthy woman who travelled across one time zone (Italy to UK and vice versa) every two weeks for over 2.5 years (i.e. 852 night of sleep data, with 80% of recorded days retained for analysis after exclusion of travel and non-UK/Italy stays; 58% of time spent in Italy), and reported early evening somnolence and unpleasantly early wake-up time in the UK, especially immediately after travelling. She therefore started to wear an actigraph to monitor rest-activity and light exposure. Compared to Italy, UK-based recordings showed a significantly earlier midsleep (02:46 ± 01:00 vs. 03:02 ± 01:07 local clock time; p < 0.01) and more wake after sleep onset (WASO, 57.4 ± 29.7 vs. 46.5 ± 30.3 min; p < 0.001). Time spent above the 1000 lux threshold, used as a reference for outdoor light exposure, was significantly greater in the UK (92 ± 82 min vs. 56 ± 78 min, p < 0.001). Higher daytime light exposure was associated with earlier midsleep and reduced sleep fragmentation, as indicated by lower WASO. In conclusion, frequent travel across one time zone resulted in prolonged sleep-wake disturbance, with no obvious pattern of resolution after the flight.

Low indoor light in urban housing can dis-

rupt health and wellbeing, especially in older adults

who experience reduced light sensitivity and sleep/cir-

cadian disruptions with natural aging. While controlled

studies suggest that enhancing indoor lighting may

alleviate the negative effects of reduced light sensitiv-

ity, 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 sub-

jective 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 col-

lected 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 ben-

eficial 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.