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Light & Sleep (Or the Lack Thereof)

Posted by Ilena Di Toro | Posted on March 10, 2020

Getting up at night to go to the bathroom or just getting up in the morning can be difficult. Especially since we’ve been asleep for hours and the first exposure to light can blinding. Of course, our eyes adjust and we go about our tasks or go back to sleep.

Exposure to light doesn’t just help us to do what we need to do each day, it also helps to set our circadian rhythm, the sleep/wake cycle. The most powerful cue is light from the sun. In fact, the ganglion cells in the eye are connected to the suprachiasmatic nucleus (SCN). This is the part of the brain that contains the biological clock that puts in motion the circadian rhythm. Daylight signals to the brain that is time to be awake and as the amount of light diminishes in the hours after sunset, the brain gets the signal to wind down and go to sleep.

Yet, how exactly does the brain respond to light signals? Specifically, how do the neurons communicate so that the sleep/wake cycle is set in motion? Researchers at the Salk Institute and UC San Diego studied this and here is what they learned.

There are neurons called intrinsically photosensitive retinal ganglion cells (ipRGCs).They are found in the retina and extract a protein called melanopsin that senses blue light. Researchers used a virus to deliver a protein so that the cells can be viewed under an electron microscope. This was done to attach the protein the light sensitive cells, so that the neuron and it long axon can be tracked.

Researchers were able to follow the processes coming from the nerve cell from the retina to the places where they connect to brain regions responsible for circadian rhythms, eye reflexes and vision. The ipRGCs connect to many parts of the brain that regulate different tasks. The cells tell one part of the brain that it is bright outside, so the pupil shrink and it does so in less than a second. The ipRGCs also connects to the brain’s master clock to regulate the sleep/wake cycle. The question that needed to be answered was how do the ipRGCs do these different tasks.

It was found that the difference has to do with the way light detected by the retina reaches the brain. Scientists were able to trace the signal to the brain region that constricts the pupil and they found that that those connections were strong like water coming out of a hose. When it comes to the ipRGCs and the master clock, the connections were weak, like water coming out of a drip irrigation system. Since the ipRGCs delivers the light to the brain’s master clock, it takes longer for information to reach it and reset the clock. That explains why it is easier to go back to sleep after going to the bathroom, but it is harder to go back to sleep after spending a half hour looking for something around the house. In the case of spending a half hour to look for something, enough light reached the brain that the clock was reset.

“These findings and methods open new opportunities for brain researchers studying the long-distance wiring of brains in normal and in animal models of human disease,” said Mark Ellisman, distinguished professor of neurosciences at UC San Diego and adjunct professor at Salk, who co-led the work.

Something to think about as you try to go back to sleep after you send that last-minute email to a work colleague at 11:30 pm.


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