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Red light improved vision after 40
The vision experiments
The experiments came from the lab of a serious vision scientist, and they covered different ages and different eye conditions. Volunteers looked at the light for brief sessions a couple of times a week. Only people older than 40 improved, and only when the sessions fell in the first three hours of the day.
Why the retina responds
The explanation is in the retina. Photoreceptors, which absorb light and convert it into electrical signals, are the most mitochondrial rich and metabolically active cells in the body. They generate a lot of downstream products from ATP, and long-wavelength light protects them from the oxidative damage that comes with that.
Time of day
The eyes, body, and brain are not the same across the day. Sensitivity to harm from bright light rises gradually after sundown, while the need for bright light in the eyes early in the day is immense.
Why do photoreceptors burn so much energy?
In darkness, rods keep ion channels open and pump ions out continuously, which costs a steady stream of ATP. That makes the retina one of the most energy-demanding tissues in the body. Photoreceptors also rebuild their light-catching outer segments constantly. Experiments in the 1960s by Richard Young showed that a rod renews roughly a tenth of its outer segment every day.
Why would the retina care about time of day?
The retina has its own circadian clock, separate from the master clock in the brain. Rods shed the tips of their outer segments shortly after light onset in the morning, and cells in the pigment layer beneath them eat the shed pieces. Cones follow a different schedule, shedding at night. Studies by Matthew LaVail in the 1970s first showed this rhythm.
How little light can the eye detect?
In 1942, Selig Hecht, Simon Shlaer, and Maurice Pirenne tested volunteers in a dark room with faint flashes. They calculated that people could see a flash when only about five to fourteen photons reached the retina. A single rod can respond to one photon. The result showed the eye operating close to the physical limit of light detection.
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