title: Human responses to bright light of different durations.
abstract: Light exposure in the early night induces phase delays of the circadian rhythm in melatonin in humans. Previous studies have investigated the effect of timing, intensity, wavelength, history and pattern of light stimuli on the human circadian timing system. We present results from a study of the duration–response relationship to phase-delaying bright light. Thirty-nine young healthy participants (16 female; 22.18±3.62 years) completed a 9-day inpatient study. Following three baseline days, participants underwent an initial circadian phase assessment procedure in dim light (<3 lux), and were then randomized for exposure to a bright light pulse (∼10,000 lux) of 0.2 h, 1.0 h, 2.5 h or 4.0 h duration during a 4.5 h controlled-posture episode centred in a 16 h wake episode. After another 8 h sleep episode, participants completed a second circadian phase assessment. Phase shifts were calculated from the difference in the clock time of the dim light melatonin onset (DLMO) between the initial and final phase assessments. Exposure to varying durations of bright light reset the circadian pacemaker in a dose-dependent, non-linear manner. Per minute of exposure, the 0.2 h duration was over 5 times more effective at phase delaying the circadian pacemaker (1.07±0.36 h) as compared with the 4.0 h duration (2.65±0.24 h). Acute melatonin suppression and subjective sleepiness also had a dose-dependent response to light exposure duration. These results provide strong evidence for a non-linear resetting response of the human circadian pacemaker to light duration.
authors.affiliation: Division of Sleep Medicine, Department of Medicine, Brigham & Women's Hospital/Harvard Medical School, 221 Longwood Avenue, Suite 438, Boston, MA 02115, USA. amchang@rics.bwh.harvard.edu
authors.forename: Anne-Marie
authors.initials: AM
authors.lastname: Chang
authors.forename: Nayantara
authors.initials: N
authors.lastname: Santhi
authors.forename: Melissa
authors.initials: M
authors.lastname: St Hilaire
authors.forename: Claude
authors.initials: C
authors.lastname: Gronfier
authors.forename: Dayna S
authors.initials: DS
authors.lastname: Bradstreet
authors.forename: Jeanne F
authors.initials: JF
authors.lastname: Duffy
authors.forename: Steven W
authors.initials: SW
authors.lastname: Lockley
authors.forename: Richard E
authors.initials: RE
authors.lastname: Kronauer
authors.forename: Charles A
authors.initials: CA
authors.lastname: Czeisler
mesh_terms: D000293:Adolescent; D000328:Adult; D002940:Circadian Rhythm / Q000502:physiology*; D005260:Female; D006801:Humans; D008027:Light; D008297:Male; D008550:Melatonin / Q000097:blood*; D012890:Sleep; D013997:Time Factors; D055815:Young Adult
chemical_list: D008550:Melatonin