Ever Wondered How the Brain Told Night From Day?

Day and Night
According to the latest research, the answer lies in your eyes.

The discovery of three cell types in the eye that detect light have enabled researchers to understand how humans tell night from day. The study marks the first direct assessment in humans of light responses from these cells, called intrinsically photosensitive retinal ganglion cells (ipRGCs) — and the implications for health are substantial.

According to the research, light detection through the cells help align the brain’s circadian rhythm to ambient light. Bright light at night interrupts the body’s normal day-night cycles, called circadian rhythms, and can trigger insomnia. 

In fact, circadian rhythms play a major role in health. Disrupted day-night cycles have even been linked to increased incidence of diseases like cancer, heart disease, obesity, depressive disorders and type 2 diabetes in people who work night shifts.

“We have become mostly an indoor species, and we are removed from the natural cycle of daylight during the day and near-complete darkness at night,” says Salk Professor Satchidananda Panda, senior author of the paper. “Understanding how ipRGCs respond to the quality, quantity, duration, and sequence of light will help us design better lighting for neonatal ICUs, ICUs, childcare centres, schools, factories, offices, hospitals, retirement homes and even the space station.”

While we have been told for years that ‘blue light’ and technology in the bedroom can effect our sleep and health. This new research helps understand how human eyes sense light and could lead to “smart” lights that can prevent depression, foster sleep at night, and maintain healthy circadian rhythms.

“It’s also going to open a number of avenues to try new drugs or work on particular diseases that are specific to humans,” says Ludovic Mure, a postdoctoral researcher in the Panda lab and first author of the new study.

While mouse retinas have been used in the past to test light response, this is the first time human retinas have been successfully used. Along with partners at the John A. Moran Eye Center of the University of Utah, researchers were able to keep the retinas samples after their donors passed away. 

They found that a small group of cells began firing after just a 30-second pulse of light. After the light was turned off, some of these cells took several seconds to stop firing. The researchers tested several colors of light, and found that these “intrinsically photosensitive” cells were most sensitive to blue light — the type used in popular cool-white LED lights and in many of our devices, such as smartphones and laptops.

Follow-up experiments revealed three distinct types of ipRGCs.

  • Type 1 responded to light relatively quickly but took a long time to turn off.
  • Type 2 took longer to turn on and also very long to turn off.
  • Type 3 responded only when a light was very bright, but they turned on faster and then switched off as soon as the light was gone.

Understanding how each ipRGC type functions may allow researchers to better design lighting or even therapeutics that can turn the cell activity on or off.

The new study actually helps explain a phenomenon reported in past studies of some blind people. These people, despite not being able to see, are still able to align their sleep-wake cycle and circadian rhythms to a day-night cycle. Thus, they must be sensing light somehow.

Now it appears that ipRGCs are the cells responsible for sending that light signal to the brain, even in people who lack the rod and cone cells needed to relay an image to the brain.



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