Abstract
Light around twilight provides the primary entrainment signal for circadian rhythms. Here we review the mechanisms and responses of the mouse and human circadian systems to light. Both utilize a network of photosensitive retinal ganglion cells (pRGCs) expressing the photopigment melanopsin (OPN4). In both species action spectra and functional expression of OPN4 in vitro show that melanopsin has a λmax close to 480 nm. 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. Studies in mice, non-human primates and humans, show that rods and cones project to and can modulate the light responses of pRGCs. 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. Although photoreceptor mechanisms are similar, sensitivity thresholds differ markedly between mice and humans. 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). The basis for this difference remains unclear. 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. 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.
Highlights
Light around twilight provides the primary entrainment signal for circadian rhythms
phase response curve (PRC) and natural light exposure is not straightforward, the easiest way to think about the delaying and advancing impact of light on the circadian system is to consider a nocturnal mouse in the wild, emerging from its burrow during early dusk
Especially the non-mammalian vertebrates, there is remarkable diversity in the light detecting mechanism whereby light is detected for circadian entrainment and masking [13,14,15,16]; the focus of this review will be confined to circadian entrainment in mice and humans
Summary
To be of any value, an endogenous circadian clock must be set to local time. The majority of circadian clocks utilize a solar-based mechanism as the primary means to synchronize (entrain) the biological day to the astronomical day. If the animal is exposed to a single one-hour pulse of light during its subjective circadian day, as shown in (A), there is usually no or little phase shifting effect on the freerunning rhythm PRC and natural light exposure is not straightforward, the easiest way to think about the delaying and advancing impact of light on the circadian system is to consider a nocturnal mouse in the wild, emerging from its burrow during early dusk Assuming it does not get eaten, the mouse will be exposed to light at a time that will delay its clock, and activity will start later the day, with the mouse emerging after dusk and reducing the risk of predation. Especially the non-mammalian vertebrates, there is remarkable diversity in the light detecting (photoreceptor) mechanism whereby light is detected for circadian entrainment and masking [13,14,15,16]; the focus of this review will be confined to circadian entrainment in mice and humans
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