They also seem to be implicated in the pathways driving the arousal and sleep-promoting effects of blue and green light, respectively, that have recently been found in mice 5. These cells are directly excited by blue light (their peak of sensitivity has been demonstrated at 480 nm, although they are also activated by other wavelengths) 3, and send the light irradiance information to the SCN through the retinohypothalamic tract (RHT) (intrinsic pathway) 4. Light enters the circadian pacemaker through the eyes, and this pathway starts in a particular type of retinal ganglion cells (ipRGCs) containing melanopsin, which makes them intrinsically photosensitive 2. The main input to keep the circadian system entrained to the external world is the light/dark cycle 1. It allows organisms to adjust their physiology and behaviour and anticipate daily environmental changes, instead of merely reacting to them. The circadian timing system, a hierarchically organised network of structures responsible for generating ~24 h or daily rhythms, is driven in mammals by a circadian pacemaker located in the suprachiasmatic nuclei (SCN) of the hypothalamus. This could be relevant for developing lighting that reduces the disruptive effects of nocturnal light in humans, without compromising chromaticity. For degus SCN c-Fos activation by light was stronger with RGB-light than with RGV. Degus remained synchronised, despite the fact that day and night-time lighting systems differed only in spectra, but not in intensity. Activity rhythms free-ran in rats under a RGB:RG cycle and became arrhythmic under RGB:RGV. Animals were subjected to combined red-green-blue lights (RGB) during the day and to: darkness red light (R) combined red-green LED (RG) lights and combined red-green-violet LED (RGV) lights during the night. This study tests the capacity of a diurnal Octodon degus and nocturnal Rattus norvegicus to synchronise to different nocturnal lights. Exposure to blue-light at night leads to circadian misalignment that could be avoided by using less circadian-disruptive wavelengths. Light intensity, spectra, and timing are important for SCN synchronisation.
The central circadian pacemaker (Suprachiasmatic Nuclei, SCN) maintains the phase relationship with the external world thanks to the light/dark cycle.
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