These differential effects of light are described by the “phase response curve” or PRC. By contrast, light delivered during the first six hours of the subjective night (CT 12–18) cause a phase delay-the animal will start its activity later the following day, whilst light exposure during the second half of the subjective night and towards morning (CT 18–24) will advance activity onset. Light delivered during subjective daytime has a minimal effect. ) Note: Circadian Time (CT) is a standardized notation of the 24 h phase of a circadian cycle representing an estimation of the organism’s subjective time. If animals are maintained under constant darkness and then exposed to a discrete pulse of light at varied times over the subjective day, the shifting (phase shifting) effects on the endogenous clock (freerunning rhythm) vary. Thus, there is a complex circadian network within an individual that is regulated by a hierarchy of zeitgebers which “fine-tune” performance to the varied demands of the solar cycle. These peripheral clocks then drive countless behavioral, physiological and biochemical outputs. Most, if not all cells within multicellular organisms possess the ability to express a circadian rhythm, and these independent clocks can be regulated by a variety of different signals. Although light is the primary zeitgeber for the circadian system of most organisms, it is not the only zeitgeber. In multicellular organisms, a master clock is usually entrained to the external light/dark cycle, and then acts in-turn to entrain multiple circadian oscillators throughout the rest of the body (peripheral clocks). Without this daily re-setting, the internal day would soon drift and be out of alignment with the environmental day/night cycle. Circadian clocks are not exactly 24 h (hence the term: circa/about and dies/day), and in this regard resemble an old mechanical grandfather clock which needs a slight daily adjustment to make sure the clock is set to the “real” astronomical day. For more than four billion years, the changes in the quality and quantity of light at twilight have been the main time-giver or “zeitgeber” that enables entrainment for life on Earth. The majority of circadian clocks utilize a solar-based mechanism as the primary means to synchronize (entrain) the biological day to the astronomical day. To be of any value, an endogenous circadian clock must be set to local time. Shedding Light on the Clock-The Phase Response Curve A way forward will be to define human circadian responses to artificial and natural light in the “real world” where light intensity, duration, spectral quality, time of day, light history and age can each be assessed.ġ. As our retinal light exposure is highly dynamic, and because photoreceptor interactions are complex and difficult to model, attempts to develop evidence-based lighting to enhance human circadian entrainment are very challenging. The basis for this difference remains unclear. Mice can entrain to light at approximately 1 lux for a few minutes, whilst humans require light at high irradiance (>100’s lux) and of a long duration (>30 min). Although photoreceptor mechanisms are similar, sensitivity thresholds differ markedly between mice and humans. Such an integration of signals enables the rods to detect dim light, the cones to detect higher light intensities and the integration of intermittent light exposure, whilst melanopsin measures bright light over extended periods of time. Studies in mice, non-human primates and humans, show that rods and cones project to and can modulate the light responses of pRGCs. Anatomical findings demonstrate that there are multiple pRGC sub-types, with some evidence in mice, but little in humans, regarding their roles in regulating physiology and behavior. In both species action spectra and functional expression of OPN4 in vitro show that melanopsin has a λ max close to 480 nm. Both utilize a network of photosensitive retinal ganglion cells (pRGCs) expressing the photopigment melanopsin (OPN4). Here we review the mechanisms and responses of the mouse and human circadian systems to light. Light around twilight provides the primary entrainment signal for circadian rhythms.
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