title: Effectiveness of evening phototherapy for insomnia is reduced by bright daytime light exposure.
abstract: 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.
authors.affiliation: Department of Psychiatry and Behavioral Sciences, Stanford University, Stanford, CA 94305, United States. jzeitzer@stanford.edu
authors.forename: Jamie M
authors.initials: JM
authors.lastname: Zeitzer
authors.forename: Leah
authors.initials: L
authors.lastname: Friedman
authors.forename: Jerome A
authors.initials: JA
authors.lastname: Yesavage
mesh_terms: D000368:Aged; D000375:Aging / Q000502:physiology; D019454:Chronotherapy / Q000379:methods* / Q000592:standards; D002940:Circadian Rhythm / Q000502:physiology; D005260:Female; D006801:Humans; D008029:Lighting; D008297:Male; D008875:Middle Aged; D017440:Photoperiod; D010789:Phototherapy / Q000379:methods* / Q000592:standards; D007319:Sleep Initiation and Maintenance Disorders / Q000503:physiopathology* / Q000628:therapy*; D013472:Sunlight; D016896:Treatment Outcome