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. 2011;6(7):e22078.
doi: 10.1371/journal.pone.0022078. Epub 2011 Jul 8.

Response of the human circadian system to millisecond flashes of light

Jamie M Zeitzer  1 , Norman F Ruby, Ryan A Fisicaro, H Craig Heller
Affiliations

Response of the human circadian system to millisecond flashes of light

Jamie M Zeitzer et al. PLoS One. 2011.

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.

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Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1
Figure 1. Spectral distribution of tungsten lamp as measured by a PR-650 SpectraScan Colorimeter (Photo Research, Chatsworth CA).
Data were normalized to maximum power and plotted over the visible spectrum.
Figure 2
Figure 2. Phase shifts following the flash or dark stimuli.
Average salivary melatonin profiles pre- (dashed lines) and post- (solid lines) exposure to either a series of flashes (left panel, 60 2-ms flashes, 1 per minute) or continuous darkness (right panel) for an hour are shown. Data were z-score transformed prior to averaging and aligned to a habitual bedtime of 24:00.
Figure 3
Figure 3. Change in EEG power in response to flash or dark stimuli.
Percentage change in EEG power from before to after the dark (black) or flash (red) stimuli are presented. Significant (*p<0.05) differences between conditions are found in the delta (δ) and sigma (σ) bands.
See this image and copyright information in PMC

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