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Ashutosh Jnawali, Benjamin T Backus, Elizabeth M Quinlan, Cristina Llerena-Law, Suresh Viswanathan, Nabin Joshi, Jason Grygier, Lisa A Ostrin; Physiological Effects of Ten Days of Total Darkness in Humans. Invest. Ophthalmol. Vis. Sci. 2017;58(8):4133.
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© ARVO (1962-2015); The Authors (2016-present)
Light exposure impacts many physiological and metabolic processes including circadian rhythm, mood, metabolism, and eye growth. Light contributes to circadian rhythm by synchronizing diurnal activity of intrinsically photosensitive retinal ganglion cells (ipRGCs), which modulate the release of the hormone melatonin. Here, we investigated the effects of total darkness on rhythm-dependent markers and the ipRGC-driven pupil response in humans.
Two subjects, ages 21 and 27, were sequestered for 10 days in complete, continual darkness. Subjects wore an actigraph device for 30 days, beginning 10 days before the initiation of visual deprivation, for objective measures of light exposure, activity and sleep. Morning and nighttime saliva samples were collected for subsequent melatonin analysis, and body temperature, heart rate and blood pressure were recorded. During sequestration, zeitgebers were provided, and subjects had access to auditory-output timepieces. Pupil responses to short and long wavelength light were measured immediately before and after sequestration as an assessment of ipRGC activity.
Actigraphs revealed a decrease in daily activity for the two subjects from 384 ± 154 activity counts per minute (mean cpm ± sd) during baseline, to 278 ± 109 cpm during dark exposure. Sleep duration increased from 389 ± 68 m to 528 ± 62 m for subject 1 (no baseline sleep data collection for subject 2). During the 10-day dark period, morning melatonin increased according to the fitted regression equation y = 1.2x + 2.4 where x was day in the dark, and nighttime melatonin decreased as y = -0.8x + 22.3. Therefore, by the end of the dark period, the morning and nighttime melatonin levels were roughly equal. The ipRGC-driven tonic pupil constriction was enhanced immediately after the dark period, compared to baseline.
Constant darkness disrupted systemic melatonin levels, increasing morning and decreasing nighttime melatonin, so that levels converged within 10 days. Physical activity decreased and sleep duration increased in the dark. However, phase shifts in the sleep/wake cycle were not observed, likely due to numerous cues, such as auditory clocks, meals, and scheduled events. We conclude that light deprivation is sufficient to impact circadian control of melatonin in humans, which may be mediated by the ipRGC pathway.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.
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