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Sleep and dreams

Our Biological Clocks

Some people who are blind can nevertheless respond to light therapy (repeated exposures to strong artificial light to help improve mood) or recognize when day breaks or night falls. These abilities are probably attributable to the retina’s photoreceptive ganglion cells. These cells can even sense the small amount of light that filters through the eyelids when they are closed.


As soon as scientists realized that the human central biological clock, located in the suprachiasmatic nuclei of the hypothalamus, needs to be reset each day by the light of the sun, they suspected that the photoreceptors in the human retina must be the starting point for this process.

It was therefore some surprise when several teams doing research on the circadian rhythm concluded, in the early 2000s, that human beings have a second light-sensitive system that is not dedicated to vision but instead is used to determine the overall intensity of the light to which the body is being exposed. This system works exactly like the light meter in a camera, which does not capture any images, but instead determines the correct exposure time according to the intensity of the available light. 

Various experiments soon showed that this newly discovered system made little or no use of the photopigments known at the time—the rhodopsin in the rods and the various opsins in the cones of the retina. Instead it used melanopsin and/or cryptochrome, two light-sensitive proteins whose role in this system is still the subject of debate.

Another surprise was that the retinal cells containing these new photosensitive pigments were neither rods nor cones, but ganglion cells. Previously, scientists had thought that the role of ganglion cells was simply to receive the signals that other retinal cells produced in response to light, then relay these signals out from the eye to the brain via the ganglion cell axons, which form the optic nerve. No one had ever suspected that some ganglion cells might have their own photoreceptive properties or electrophysiological properties adapted to detecting the intensity of ambient light.

These properties are found in a particular group of ganglion cells that have long dendrites covering wide portions of the retina and axons (identified with an arrow in the photograph to the right) that project to the suprachiasmatic nuclei of the hypothalamus.

Mouse retina ganglion cell projecting an axon (marked by the arrow) to the suprachiasmatic nucleus

Source: Ralph Nelson,

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