Circadian light.

Lighting Research Center, Rensselaer Polytechnic Institute, 21 Union Street, Troy, NY 12180, USA. .
Journal of Circadian Rhythms 02/2010; 8(1):2. DOI: 10.1186/1740-3391-8-2
Source: DOAJ

ABSTRACT The present paper reflects a work in progress toward a definition of circadian light, one that should be informed by the thoughtful, century-old evolution of our present definition of light as a stimulus for the human visual system. This work in progress is based upon the functional relationship between optical radiation and its effects on nocturnal melatonin suppression, in large part because the basic data are available in the literature. Discussed here are the fundamental differences between responses by the visual and circadian systems to optical radiation. Brief reviews of photometry, colorimetry, and brightness perception are presented as a foundation for the discussion of circadian light. Finally, circadian light (CLA) and circadian stimulus (CS) calculation procedures based on a published mathematical model of human circadian phototransduction are presented with an example.

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    ABSTRACT: OBJECTIVE: To examine, in a field study circadian phase changes associated with two different light-dark exposures patterns, one that was congruent with a phase advanced sleep schedule and one that was incongruent with an advanced schedule. METHODS: Twenty-one adults (mean age±standard deviation=22.5±3.9years; 11 women) participated in the 12day study. After a five-day baseline period, participants were all given individualized, fixed, 90-minute advanced sleep schedules for one week. Participants were randomly assigned to one of two groups, an advance group with a light-dark exposure prescription designed to advance circadian phase or a delay group with light-dark exposure prescription designed to delay circadian phase. The advance group received two morning hours of short-wavelength (blue) light (λmax≈476±1nm, full-width-half-maximum≈20nm) exposure and three evening hours of light restriction (orange-filtered light, λ<525nm=0). The delay group received blue light for three hours in the evening and light restriction for two hours in the morning. Participants led their normal lives while wearing a calibrated wrist-worn light exposure and activity monitor. RESULTS: After seven days on the 90-minute advanced sleep schedule, circadian phase advanced 132±19 minutes for the advance group and delayed 59±7.5 minutes for the delay group. CONCLUSIONS: Controlling the light-dark exposure pattern shifts circadian phase in the expected direction irrespective of the fixed advanced sleep schedule.
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    ABSTRACT: A model of circadian phototransduction was published in 2005 to predict the spectral sensitivity of the human circadian system to narrow-band and polychromatic light sources by combining responses to light from the spectral-opponent "blue" versus "yellow" cone bipolar pathway with direct responses to light by the intrinsically photosensitive retinal ganglion cells. In the model, depolarizing "blue" responses, but not hyperpolarizing "yellow" responses, from the "blue" versus "yellow" pathway are combined with the intrinsically photosensitive retinal ganglion cell responses. Intrinsically photosensitive retinal ganglion cell neurons are known to be much slower to respond to light than the cone pathway, so an implication of the model is that periodic flashes of "blue" light, but not "yellow" light, would be effective for stimulating the circadian system. A within-subjects study was designed to test the implications of the model regarding retinal exposures to brief flashes of light. The study was also aimed at broadening the foundation for clinical treatment of circadian sleep disorders by delivering flashing light through closed eyelids while people were asleep. In addition to a dark control night, the eyelids of 16 subjects were exposed to three light-stimulus conditions in the phase delay portion of the phase response curve while they were asleep: (1) 2-second flashes of 111 W/m(2) of blue (λmax ≈ 480 nm) light once every minute for 1 hour, (2) 131 W/m(2) of green (λmax ≈ 527 nm) light, continuously on for 1 hour, and (3) 2-second flashes of the same green light once every minute for 1 hour. Inferential statistics showed that the blue flash light-stimulus condition significantly delayed circadian phase and significantly suppressed nocturnal melatonin. The results of this study further our basic understanding of circadian phototransduction and broaden the technical foundations for delivering light through closed eyelids during sleep for treating circadian sleep disorders.
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    ABSTRACT: BACKGROUND: Human circadian rhythms are regulated by the interplay between circadian genes and environmental stimuli. The influence of altered sleep-wake schedules or light on human circadian gene expression patterns is not well characterized. METHODS: Twenty-one young adults were asked to keep to their usual sleep schedules and two blood samples were drawn at the end of the first week from each subject based on estimated time of dim light melatonin onset (DLMO); the first sample was obtained one and a half hours before the estimated DLMO and the second three hours later, at one and a half hours after the estimated DLMO. During the second week, participants were randomized into two groups, one that received a one hour blue-light (λmax=470nm) exposure in the morning and one that received a comparable morning dim-light exposure. Two blood samples were obtained at the same clock times as the previous week at the end of the second week. RESULTS: We measured the expression of 10 circadian genes in response to sleep-wake schedule advancement and morning blue-light stimulation in the peripheral blood of 21 participants during a two-week field study. We found that nine of the 10 circadian genes showed significant expression changes from the first to the second week for participants in both the blue-light and dim-light groups, likely reflecting significant advances in circadian phase. CONCLUSIONS: This wholesale change in circadian gene expression may reflect considerable advances in circadian phase (i.e., advance in DLMO) from the first to the second week resulting from the advanced, daily personal light exposures.
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