Our bodies are tuned to the cycle of the sun. We evolved to experience sunlight during the day and darkness during the night. All of our major organ systems are affected by this daily rhythm, called the circadian rhythm. When our bodies experience patterns of light exposure different from the expected cycle, physiological processes get disrupted, leading to a variety of negative health outcomes .
Our daily rhythms are influenced by both the amount and the color of light we receive. White light, like the light we get from the sun, is a mixture of all the different colors. But the mixture varies throughout the day, containing more blue light in the morning and more red light in the evening. Bright white LED light bulbs contain a similar blue-rich color profile to the morning and mid-day sunlight, while soft white, or yellowish-looking incandescent light bulbs better resemble the red-rich color profile of the evening sunlight. Special cells in our eyes help tune our internal rhythms to the pattern of light and darkness we experience. These cells are especially sensitive to blue light, such that they promote wakefulness in response to blue-rich morning sunlight and restfulness when blue light levels decline.
The mechanisms that promote learning and memory are influenced by this cycle. Our brains are primed to learn new information during the day when there is a lot of light, and then to store it in long-term memory during the dark night as we sleep. Student performance increases when classrooms are illuminated with bright (blue-rich) lights . Bright light exposure activates regions of the brain that promote alertness, and improves cognitive performance . Light activates factors that are essential for memory formation , as well as factors that are important for the regulation of mood and overall brain health . Studies have shown that when animals are exposed to constant dim light, instead of the natural cycle of bright light and darkness, they show impairments in learning and memory, as well as decreased levels of brain protective factors .
Unfortunately, many of us have become like the study animals exposed to constant dim light, instead of the natural cycle. Today, 99% of people in the USA experience some form of light pollution, or exposure to unnaturally high levels of light during the evening, from both indoor and outdoor sources . As we spend more time indoors under dim artificial lights, we are also experiencing less of the high-intensity bright sunlight. For reference, sunlight is on the order of 10,000 lux, with lux being a measure of light intensity, while the full moon is on the order of 0.1 lux [1; 6]. The levels inside buildings tends to range from 100 to 1000 lux, with older incandescent lighting systems being considerably dimmer than newer LED-based lighting. Levels of evening outdoor light in residential areas is around 10 lux. This means we are getting at least 1000 times less light during the day and at least 100 times too much light at night. Additionally, the color distribution of most artificial lights is constant.
Mistimed light exposure disrupts hormone production, and can lead to an elevated stress response and mood disorders . Evening light suppresses the production of the sleep promoting hormone melatonin, leading to insomnia and other sleep disorders. Individuals with dementia are particularly affected by disruptions to the circadian rhythm, which can exacerbate depression and agitation.
WHAT YOU CAN DO
There are efforts underway to better design our living spaces with lighting that is more in tune with our biology . Until some of these practices become widespread, we may not have much control over light pollution stemming from the outdoor environment. However, we can make changes to our daily routine and exposure to indoor light. Pilot studies have found that the implementation of these strategies, namely bright light therapy in the morning and reduced light exposure during the evenings, improved sleep consolidation and mood, while reducing agitation in patients with dementia [9; 10].
1. Get more bright blue-rich light during the morning
2. Minimize light exposure, especially blue light, during the evening
Adopting these practices to reset our rhythms back in tune with the natural sun cycle can improve our overall health and sense of well-being throughout life.
Betsy Mills, PhD, is a member of the ADDF's Aging and Alzheimer's Prevention program. She critically evaluates the scientific evidence regarding prospective therapies to promote brain health and/or prevent Alzheimer's disease, and contributes to CognitiveVitality.org.
Dr. Mills came to the ADDF from the University of Michigan, where she served as the grant writing manager for a clinical laboratory specializing in neuroautoimmune diseases. She also completed a Postdoctoral fellowship at the University of Michigan, where she worked to uncover genes that could promote retina regeneration. She earned her doctorate in neuroscience at Johns Hopkins University School of Medicine, where she studied the role of glial cells in the optic nerve, and their contribution to neurodegeneration in glaucoma. She obtained her bachelor's degree in biology from the College of the Holy Cross. Dr. Mills has a strong passion for community outreach, and has served as program presenter with the Michigan Great Lakes Chapter of the Alzheimer's Association to promote dementia awareness.
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