Abstract
Sleep problems increase with age and this may be, in part, due to a reduction in environmental and perceived light levels and the impact these have on the circadian system and daytime functioning. This study investigated the effects of daytime light exposure delivered as a skeleton photoperiod on subjective and actigraphic sleep, daytime mood and alertness, circadian activity rhythms and melatonin production and timing in healthy older people (>60 years) with self-reported sleep problems (Pittsburgh Sleep Quality Index >5). Two polychromatic light sources with different spectral compositions were compared, a blue-enriched (17000 K) and a control (4000 K) white light condition, at two different irradiances. Participants (66.5 +- 4.7 years; 23F, 10M) completed an at-home study of 11 consecutive weeks: 1 week baseline followed by 3 weeks daily light exposure (2 h in the morning and 2 h evening) and 2 weeks of washout for each light condition (randomised, crossover design). Twelve participants received low irradiance lights (-3.6x1014 photons/cm2/sec, ~400 lux) and 21 participants received high irradiance lights (~9.1x1014 photons/cm2/sec, ~1100 lux). Participants completed daily sleep diaries, mood and alertness scales, and wore an activity monitor continuously. The urinary metabolite of melatonin, 6-sulphatoxymelatonin, was measured before and at the end of each light exposure period (sequential urine collection over a 39 h period). Timed light exposure showed some beneficial effects on subjective and actigraphic sleep, mood, alertness, activity and the melatonin rhythm. Results were shown to be irradiance- and period-dependent. Blue-enriched light significantly delayed subjective and actigraphic sleep time compared to control light. Light administration produced few effects, possibly due to insufficient strength of the photic signal (irradiance, duration) as well as the confounding effects of a real world-life environment (natural photoperiod, social commitments). Future work should focus on refining and optimising light treatment for older community-dwelling individuals with sleep problems.