Abstract
Ocular light sensitivity is the primary mechanism by which the central circadian clock, located in the suprachiasmatic nucleus (SCN), remains synchronized with the external geophysical day. This process is dependent on both the intensity and timing of the light exposure. Little is known about the impact of the duration of light exposure on the synchronization process in humans. In vitro and behavioral data, however, indicate the circadian clock in rodents can respond to sequences of millisecond light flashes. In a cross-over design, we tested the capacity of humans (n = 7) to respond to a sequence of 60 2-msec pulses of moderately bright light (473 lux) given over an hour during the night. Compared to a control dark exposure, after which there was a 3.5±7.3 min circadian phase delay, the millisecond light flashes delayed the circadian clock by 45±13 min (p<0.01). These light flashes also concomitantly increased subjective and objective alertness while suppressing delta and sigma activity (p<0.05) in the electroencephalogram (EEG). Our data indicate that phase shifting of the human circadian clock and immediate alerting effects can be observed in response to brief flashes of light. These data are consistent with the hypothesis that the circadian system can temporally integrate extraordinarily brief light exposures.
Highlights
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]
We examined the capacity of the human hypothalamus to respond to millisecond flashes of light
In the remaining six subjects, there was no difference in the interval between the onset of melatonin and the beginning of the two experimental light stimulation protocols (5 h 9 min650 min vs. 4 h 59 min665 min, flash vs. dark; p = 0.64, paired t-test), indicating that the two experimental light exposures were given at the same circadian phase in all subjects
Summary
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]. 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 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]. We examined the capacity of the human hypothalamus to respond to millisecond flashes of light
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