Wearing orange-tinted glasses in the evening has been shown to improve sleep patterns because they block certain wavelengths of light emitted by electronic screens.
Devices such as smartphones and computers are often illuminated by light-emitting diodes (LEDs), which tend to emit more blue light than incandescent bulbs. Light from the blue part of the spectrum inhibits production of melatonin, a hormone that helps people fall asleep, so looking at these devices before bedtime can reduce sleepiness. Light intensity is also important, so that LED televisions have less of an effect because they are viewed from further away, as do the backlit screens of some e-book readers.
A study of teenage boys who looked at their screens for a few hours before bedtime found that those who wore orange-tinted glasses felt “significantly more sleepy” than those who wore clear glasses. Another study found that two hours of exposure to a bright tablet computer screen at night reduced melatonin levels by about 22%. And men exposed to fluorescent bulbs have been shown to produce 40% less melatonin than when they were exposed to incandescent bulbs.
Ageing reduces these effects, but can be responsible for a whole set of other light-related problems. Ageing increases crystalline lens light absorption and decreases pupil area, resulting in progressive loss of photoreception. This means that a 10 year-old child has 10 times greater photoreception than a 95 year-old. This decreased sensitivity to blue light means that older people are less likely to be kept awake at night by computer screens.
But because melatonin suppression improve alertness, cognition and mood, older people’s reduced blue light sensitivity may be at least partly responsible for memory loss, slower reaction times and depression. For similar reasons, removal of clouded lenses during cataract surgery has been shown to reduce insomnia and daytime sleepiness and to improve reaction times.
Until recently, rods and cones in the eye were thought to be its only photoreceptors, but retinal ganglion cells are now known to also be involved. These cells are particularly responsive to blue light, and transmit messages to the suprachiasmatic nucleus in the brain which controls the body’s 24-hour circadian rhythms via the release of melatonin and cortisol.
Disruption of circadian rhythms can not only affect sleep but may contribute to conditions such as heart disease and cancer. The increasing use of LEDs and more energy-efficient fluorescent bulbs, which emit more blue light than older incandescent bulbs, may therefore trigger more health issues in the future.