title: Circadian phase resetting in older people by ocular bright light exposure.
abstract: BACKGROUND
  Aging is associated with frequent complaints about earlier bedtimes and waketimes. These changes in sleep timing are associated with an earlier timing of multiple endogenous rhythms, including core body temperature (CBT) and plasma melatonin, driven by the circadian pacemaker. One possible cause of the age-related shift of endogenous circadian rhythms and the timing of sleep relative to clock time is a change in the phase-shifting capacity of the circadian pacemaker in response to the environmental light-dark cycle, the principal synchronizer of the human circadian system.
  
  
  METHODS
  We studied the response of the circadian system of 24 older men and women and 23 young men to scheduled exposure to ocular bright light stimuli. Light stimuli were 5 hours in duration, administered for 3 consecutive days at an illuminance of approximately 10,000 lux. Light stimuli were scheduled 1.5 or 3.5 hours after the CBT nadir to induce shifts of endogenous circadian pacemaker to an earlier hour (phase advances) or were scheduled 1.5 hours before the CBT nadir to induce shifts to a later hour (phase delays). The rhythms of CBT and plasma melatonin assessed under constant conditions served as markers of circadian phase.
  
  
  RESULTS
  Bright light stimuli elicited robust responses of the circadian timing system in older people; both phase advances and phase delays were induced. The magnitude of the phase delays did not differ significantly between older and younger individuals, but the phase advances were significantly attenuated in older people.
  
  
  CONCLUSIONS
  The attenuated response to light stimuli that induce phase advances does not explain the advanced phase of the circadian pacemaker in older people. The maintained responsiveness of the circadian pacemaker to light implies that scheduled bright light exposure can be used to treat circadian phase disturbances in older people.
authors.affiliation: Circadian, Neuroendocrine, and Sleep Disorders Section, Division of Endocrinology-Hypertension, Department of Medicine, Harvard Medical School and Brigham and Women's Hospital, Boston, Mass., USA. ebklerman@hms.harvard.edu
authors.forename: E B
authors.initials: EB
authors.lastname: Klerman
authors.forename: J F
authors.initials: JF
authors.lastname: Duffy
authors.forename: D J
authors.initials: DJ
authors.lastname: Dijk
authors.forename: C A
authors.initials: CA
authors.lastname: Czeisler
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000375:Aging; D002940:Circadian Rhythm / Q000528:radiation effects*; D005260:Female; D006801:Humans; D008027:Light; D008297:Male; D010775:Photic Stimulation; D017440:Photoperiod; D012890:Sleep / Q000528:radiation effects