It's not every day that a news story about circadian rhythms shoots to the top of the "most read" list in a major newspaper. But that's exactly what happened with this story about how the aging of our eyes may affect our circadian clocks and in turn, our overall health. This New York Times piece outlines research that is working to connect the aging of the eye with disruption to the functioning of the body's circadian clocks, and the development of many of the chronic and serious health conditions that are associated with age.
Circadian rhythms—our own internal biological "clock"—govern our sleep-wake cycle and several other daily rhythms of the body. It's a complicated and finely tuned system of hormonal and bio-chemical reactions that helps us live in rhythm with the 24-hour day, waking in the morning and remaining alert throughout the daylight hours, then sleeping and rejuvenating at night. A key component of our circadian system is the timed release of the "sleep hormone" melatonin. When our circadian clocks are functioning properly, melatonin levels rise during the nighttime hours, promoting sleep. Melatonin levels are suppressed during daytime hours, as other hormones such as cortisol rise, helping to provide the alertness and energy we need to navigate our waking day.
Our bodies' circadian clocks, and the rise and fall of melatonin, are driven by exposure to light. And our eyes play a critical role in capturing that light and transmitting information to the part of the brain that governs our circadian clocks.
Scientists in recent years have discovered a group of cells in the retina of the eye that are triggered by light to communicate with the area of the brain that controls the circadian clock. Researchers have determined that these light-sensitive retinal cells communicate directly with the brain's suprachiasmatic nucleus. This small cell cluster, located in the hypothalamus, is responsible for controlling the body's circadian rhythms.
Our circadian clocks are incredibly precise, and can be very sensitive to disruption:
Exposure to light at night. This happens most often these days through our ever-present electronic devices, which seem to be everywhere, including the bedroom. Falling asleep with the television on, leaving a cell phone, laptop or tablet on your bedside table—the light emitted from these devices can throw your circadian rhythms out of whack and disrupt your sleep.
Shift work. People who work nights, or an irregular combination of day and night shifts, are frequently living at odds with their biological clocks. Firefighters, police, medical professionals, pilots and other shift workers, as well as people who travel frequently across time zones, suffer sleeplessness and other health problems as a result of disruptions to their circadian clocks.
Here's where the aging of the eye comes into play. As our eyes age, they become less effective at absorbing light. Pupils become narrower. The lens of the eye takes on a yellow cast. Overall, our eyes become less adept at absorbing light-particularly blue light, the part of the light spectrum that has an especially powerful effect on the retinal cells that work with the brain to control circadian rhythm. This study showed how dramatically blue-light absorption decreases with age:
• The eyes of a 10 year old have 10 times the ability to absorb blue light as the eyes of a 95 year old
• By age 45, a person's eyes are able to absorb only 50% of the blue light needed to keep circadian rhythms functioning properly
Research into the effects of blue light exposure on the body's circadian rhythms reveals the negative consequences of the decrease in blue-light absorption that comes with age:
• In this study, women were exposed to blue light for a continuous 30-minute period. The younger women in the study responded to the blue light exposure with a drop in melatonin levels. Older women, exposed to the same amount of blue light, did not experience a suppression of melatonin levels.
• This research revealed that older men were significantly less affected by exposure to blue light than younger men, when researchers measured for alertness and mood. Younger men experienced a boost in alertness and mood after exposure to blue light that older men simply did not experience.
What does all this fascinating science mean in practical terms? Exposure to light, especially sunlight, matters. And it matters increasingly as we age. As our ability to use light effectively decreases, we need to increase the amount of our exposure in order to help strengthen and boost our circadian rhythms. Keeping our eyes healthy and promptly treating eye problems such as cataracts is critical. The health of our eyes, it seems, may have a profound effect on the quality of our sleep and our overall health.
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