Objective: To examine the effect of ambulatory daytime light exposure on phase delays and on the advances produced by timed exposure to bright evening or morning light.
Methods: As a subset of a larger study, 32 older (63.0 ± 6.43 years) adults with primary insomnia were randomized to an at-home, single-blind, 12-week, parallel-group study entailing daily exposure to 45 min of scheduled evening or morning bright (∼4000 lux) light. Light exposure patterns during the baseline and the last week of treatment were monitored using actigraphs with built-in illuminance detectors. Circadian phase was determined through analysis of in-laboratory collected plasma melatonin.
Results: Less daytime light exposure during the last week of treatment was significantly associated with larger phase delays in response to evening light (r's>0.78). Less daytime light exposure during the last week of treatment was also associated with a significant delay in wake time (r's>-0.75). There were no such relationships between light exposure history and phase advances in response to morning light.
Conclusions: Greater light exposure during the daytime may decrease the ability of evening light, but not morning light, exposure to engender meaningful changes of circadian phase.
Published by Elsevier B.V.