In depth morphological analyses allowed for an ipRGC population clustering into five distinct subtypes, which moreover express a certain diversity in their dendritic stratification, morphology, membrane properties, and melanopsin expression levels 7. The melanopsin containing retinal ganglion cells, called intrinsic photosensitive retinal ganglion cells (ipRGCs) count for around 1% of the total RGCs and are found in front of the retina (see Figure 1) to process and send signals 3- 5 among others to the suprachiasmatic nucleus (SCN), the so called master clock of the brain 6. Importantly, these new photoreceptors are retinal ganglion cells (RGCs) and communicate directly with the brain. It was not until 1998, when a new type of photoreceptor in the human eye was discovered 2 and later identified to be especially sensitive to blue light. The ability of the visible light to synchronize the biological clock of the photosynthetic marine dinoflagellate Gonyaulax polyedra to the solar 24-hour cycle is known since 1958 1. ![]() Body temperature, hormonal levels, sleep duration and quality, cognitive performance and countless other physiological variables exhibit such daily oscillations. This rises inevitably the need for solutions to improve wellbeing, alertness, and cognitive performance in today's modern society where exposure to blue light emitting devices is ever increasing.ġ THE ADVENT OF THE CIRCADIAN PHOTORECEPTORĬircadian rhythms are biological cycles that have about 24-hour periods. While the exposure to blue is important for keeping organism's wellbeing, alertness, and cognitive performance during the day, chronic exposure to low-intensity blue light directly before bedtime, may have serious implications on sleep quality, circadian phase and cycle durations. ![]() Exposure to blue light during the day is important to suppress melatonin secretion, the hormone that is produced by the pineal gland and plays crucial role in circadian rhythm entrainment. The beneficial effect on circadian synchronization, sleep quality, mood, and cognitive performance depends not only on the light spectral composition but also on the timing of exposure and its intensity. Circadian rhythm is important for optimum function of organisms and circadian sleep–wake disruptions or chronic misalignment often may lead to psychiatric and neurodegenerative illness. ![]() Short wavelengths, perceived as blue color, are the strongest synchronizing agent for the circadian system that keeps most biological and psychological rhythms internally synchronized. Visible light synchronizes the human biological clock in the suprachiasmatic nuclei of the hypothalamus to the solar 24-hour cycle.
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