Submitted on April 4, 2005
Accepted on May 6, 2005
Temporal dynamics of late night photic stimulation of the human circadian timing system
Jamie M Zeitzer1, Sat Bir S Khalsa2, Diane B Boivin3, Jeanne F Duffy2, Theresa L Shanahan2, Richard E Kronauer4, and Charles A Czeisler2*
1 Stanford Center for Narcolepsy Research, Stanford University Medical Center, Palo Alto, CA, USA
2 Division of Sleep Medicine, Department of Medicine, Harvard Medical School, Brigham and Women's Hospital, Boston, MA, USA
3 Douglas Hospital Research Centre, McGill Medical School, Verdun, Qc, Canada
4 Division of Applied Sciences, Harvard University, Cambridge, MA, USA
* To whom correspondence should be addressed. E-mail: caczeisler{at}rics.bwh.harvard.edu.
The light/dark cycle is the primary synchronizing factor that keeps the internal circadian pacemaker appropriately aligned with the environmental 24 h day. Though it is known
that ocular light exposure can effectively shift the human circadian pacemaker, and do so in an intensity-dependent manner, the curve that describes the relationship between light intensity and pacemaker response has not been fully characterized for light exposure in the late biological night. We exposed subjects to three consecutive days of 5 h of experimental light, centered 1.5 h after the timing of the fitted minimum of core body temperature, and show that such light can phase advance shift the human circadian
pacemaker in an intensity-dependent manner, with a logistic model best describing the relationship between light intensity and phase shift. A similar sigmoidal relationship is also observed between light intensity and the suppression of plasma melatonin concentrations that occurs during the experimental light exposure. As with a simpler, 1-day light exposure during the early biological night, our data indicate that the human circadian pacemaker is highly sensitive even to typical room light intensities during the late biological night, with ~100 lux evoking half of the effects observed with light ten
times as bright.
