Beneficial blue light is so named because it’s essential in the daytime to keep us awake, alert and happy. But exposing our eyes to these blue wavelengths (approximately 460 to 490 nm) too close to bedtime suppresses melatonin, our sleep hormone, and that’s bad. When our melatonin levels fail to rise in preparation for bedtime, we have trouble falling asleep, staying asleep and reaching deep restorative sleep. Sleep disruption and deprivation have harmful consequences for our physical and mental health, ranging from obesity to diabetes to mood disorders.
Exposure to beneficial blue light is good in the daytime since it awakens us in the morning and keeps us alert and happy throughout the day by raising our serotonin, dopamine and other happy brain chemicals. But these brain chemicals need to turn off as the body prepares for sleep. In nature, our sleep hormones naturally rise after sunset as illuminance levels dim, and the blue light fades from our environment, making us sleepy. But since the advent of artificial indoor lighting, luminance levels and blue light floods our evening environments, and modern LED lighting and digital device screens are rich in melatonin-suppressing blue wavelengths.
We have specialized retinal ganglion cells containing the photopigment melanopsin that triggers light responses and relays irradiance information to the brain. They are called intrinsically photosensitive retinal ganglion cells or ipRGCs, and they mediate pupillary light reflex and the synchronization of our circadian clock. They also perform image-forming tasks, such as contrast optimization. These cells are sensitive to bright luminance close to bedtime and will suppress the melatonin production we need for healthy sleep. But the melanopsin’s peak sensitivity is to longer wavelengths of blue-turquoise light, between 460 and 490 nm. When these wavelengths are present in our environment too close to bedtime, they will continue suppressing melatonin, disrupting healthy sleep patterns. In my column from the November/December 2022 issue, I covered the harmful effects of sleep disruption on physical and mental health. Special note: This article does not address HEV short wavelength blue lights’ adverse effects from sunlight, nor does it address the visual effects of these short blue wavelengths on visual acuity and comfort. But to address the most serious threat to vision, patients should be made aware of the need to protect their eyes from the solar UVR and HEV blue light when outdoors. For more on HEV blue light, see our CE course, “Blue Light Lenses – 5 Reasons for Rising Demand” at 2020mag.com/ce.
• Deborah Kotob
Pro to Pro Director