The light sensitivity of the eye’s daytime color vision system evolved later than nonvisual light detection systems and so is not optimized for detecting light to reset the brain’s master circadian clock. On the visible light spectrum, the sensitivity of the three-cone photopic visual system peaks in the mid-wavelength green range, at 555 nanometers. But the nonvisual system contains the photopigment called melanopsin, which has a peak sensitivity at 480 nanometers in the short-wavelength blue range.
In animal models, and studies of different types of blindness, if the visual rods-and-cones system was removed from the cell layers of the eye, or is non-functioning, the nonvisual system works perfectly fine on its own, sending clock-setting light to the brain.
While there is some interaction, circadian neuroscientist Steven W. Lockley explains, the visual and non-visual photoreceptor systems can function independently and have their own photoreceptors, neural pathways and effects on the brain.
It’s crucial that we understand human physiology and our complex relationship with light given how important light is to sleep, circadian rhythms, and health. The eye’s nonvisual retinal system — a sci-fi-esque mechanism of which ophthalmologists largely remain unaware 13 years after the system’s discovery — represents a distinct photoreceptor systems with its own photopigment, neural pathway and function from the photoreception system that we use to see.
Neuroscientist George C. Brainard, director of Thomas Jefferson University’s Light Research Program, was one of the first scientists to study the effects of light wavelength on circadian photoreception. In 2001, his was one of two laboratories that made the major finding about the human eye: the melatonin suppression response — one of the non-visual responses to light — had a peak sensitivity in the blue light range that did not match the light sensitivity pattern of the rods and cones used to see.
Earlier work had shown that total visual blindness did not change circadian light responses, but these papers (Brainard et al., 2001; Thapan et al., 2001) provided more formal functional evidence of a non-rod, non-cone photoreceptor in the human eye.
This discovery, coming on the heels of colleagues’ research showing that the mammalian eye contains a light sensor separate from the visual rods-and-cones system, set up neuroscientist David Berson’s monumental finding in 2002: the mystery photoreceptor and a light-sensitive molecule called melanopsin, discovered in 1998 in the camouflaging skin cells of the African clawed frog, were one and the same.
Unfortunately, the conversation about light wavelengths gets stuck on the short end of the spectrum. It is true that the eye’s nonvisual light-sensing photoreceptor system is most sensitive to short-wavelength blue light, but as Lockley explains, all visible light can affect circadian rhythms. Any light source after dusk can be considered unnatural, including the light necessary to do shiftwork but also the light inside our homes, such as from TVs, cellphones, computers, and other electronic devices, which keeps us awake at night and disrupts our sleep and circadian rhythms.
Light after dusk, as relayed by the eye, tells the brain it’s daytime. And if the brain thinks it’s day, not night, it will induce daytime physiology as it thinks that we are awake at the wrong time. Consequently, light at night shifts the clock, suppresses melatonin, increases heart rate and temperature and alerts the brain — all of which are associated with daytime in a day-active species like humans.
But short-wavelength blue light — a stimulant, as opposed to relaxing, long-wavelength yellow/red light at the far end of the visible light spectrum — is also medicinal light. It can be used to improve alertness, reset disordered clock rhythms, or alleviate seasonal depression.
It’s the natural color of a brilliant, midday sky pouring in through the windows of assisted-care facilities for the elderly, improving patients’ moods, sleep, and cognitive symptoms of dementia.
And it’s a crucial component of the LED lighting wavelength model being tested for the International Space Station.