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About the Authors:
Jamie M. Zeitzer
* E-mail: [email protected]
Affiliations Department of Psychiatry and Behavioral Sciences, Stanford University, Stanford, California, United States of America, Mental Illness Research, Education and Clinical Center, VA Palo Alto Health Care System, Palo Alto, California, United States of America
Norman F. Ruby
Affiliation: Department of Biology, Stanford University, Stanford, California, United States of America
Ryan A. Fisicaro
Affiliation: School of Humanities and Sciences, Stanford University, Stanford, California, United States of America
H. Craig Heller
Affiliation: Department of Biology, Stanford University, Stanford, California, United States of America
Introduction
Exposure to bright light at night has multiple effects on the human hypothalamus, including phase shifting circadian rhythms, altering hormone production, and enhancing alertness [1]. The effects of light are dependent on both the intensity [2], [3] and timing [4] of light exposure. Transduction of light signals from the retina to the central circadian clock, located in the hypothalamic suprachiasmatic nucleus (SCN), is mediated through a network of retinal cones, rods, and melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGC) [5]. This network also projects to other hypothalamic regions [5]. To examine light-induced changes in human hypothalamic function, typical protocols use hours or, occasionally, minutes of bright light exposure [1]. Behavioral data from nocturnal rodents [6]–[8] suggest, however, that the human hypothalamus might be able to respond to light that is thousands of times shorter. As such, we examined the capacity of the human hypothalamus to respond to millisecond flashes of light.
Methods
Ethics Statement
This study and all related procedures described herein were reviewed and approved by the Stanford University Institutional Review Board and conform to the principles expressed in the Declaration of Helsinki. Subjects signed informed consent forms prior to any procedures.
Seven healthy adults (aged 18–48 years; 6 male, 1 female) participated in a pair of two-day in-laboratory sessions that were separated by at least two weeks. Subjects had no active disease processes; they were non-smokers and had normal hearing. Subjects did not have sleep disorders (Pittsburgh Sleep Quality Index score ≤5 [9]) nor did they routinely take medication that could impact their sleep, including daily use of antihistamines or antidepressants. Subjects were of intermediate chronotype as determined by the Horne-Östberg questionnaire [10].
For the two weeks prior to coming into...