An astronaut views sunrise on Earth from the International Space Station.Credit: NASA

Like many other institutional structures assembled at the end of the twentieth century, the International Space Station (ISS) was designed to incorporate fluorescent light bulbs. At present, the spacecraft is more than halfway through a lighting overhaul, and its original bulbs are being replaced, piece by piece, with light-emitting diodes (LEDs).

Part of Nature Outlook: The eye

Compared with conventional incandescent or fluorescent bulbs, LEDs use less energy, last longer and contain no glass or mercury, negating the risk of glass shards or toxic metal floating through the space station should the bulbs break in zero gravity. But researchers also hope that the new lighting system will help astronauts to sleep better at night and to stay alert during the day.

The problem that engineers are trying to address is that there’s no ‘day’ or ‘night’ in space. The ISS circles Earth every 90 minutes or so, which provides astronauts with frequent opportunities to see the Sun rise and set, but also wreaks havoc on the body’s roughly 24-hour, or circadian, clock. Among space flight’s many deleterious effects on health, disturbance of the circadian rhythm and the sleep deprivation that accompanies it have emerged as considerable worries — particularly as people contemplate travelling to more distant locations in the Solar System, says George Brainard, director of the Light Research Program at Thomas Jefferson University in Philadelphia, Pennsylvania.

The LED-based lighting system being introduced to the ISS is designed to target not just rods and cones — photoreceptor cells in the eye that enable vision in dim light and in colour, respectively — but also a third type of photoreceptor cell that was discovered almost 20 years ago. Known as intrinsically photosensitive retinal ganglion cells (ipRGCs), these photoreceptors contain a light-sensitive protein called melanopsin. They don’t have much of a role in vision; instead, ipRGCs serve as the body’s main entry point for light that regulates biological functions such as the sleep–wake cycle, alertness and mood. Researchers are beginning to understand the extent to which too much or too little light at the wrong time of day can throw important physiological processes out of sync, whether you’re an astronaut on a spacecraft, a nurse on the night shift, or just playing computer games after bedtime.

Artificial lighting has extended the length of time for which people are exposed to light each day, for better or for worse. LED-based dynamic lighting systems that are capable of adjusting the colour and intensity of the light that they deliver should make it possible to design lit environments that are less detrimental to health. “There’s no limit to the technology in terms of what can be done with LED lights,” says Robert Lucas, a neuroscientist at the University of Manchester, UK, who studies the visual system’s response to light. “That puts the onus on us, as biologists, to tell the lighting engineers exactly what they should be doing.”

Erasing the night

For thousands of years, people’s days were ruled by the rising and setting of the Sun, with help from fire to extend waking times into the evenings. Then came US inventor Thomas Edison. The carbon-filament bulb, which he patented in 1880, enabled people to keep daytime activities running around the clock and cemented incandescent lighting as a cornerstone of modern life.

Yet Edison could not have foreseen the havoc that the light bulb would wreak on people’s circadian clocks. “The combination of the 24-hour economy and the availability of electric light has led us to disregard our species’ diurnal nature,” says Luc Schlangen, a lighting scientist at Signify, an LED lighting company in Eindhoven, the Netherlands.

By the 1990s, many researchers had begun to suspect that there was more to vision than rods and cones. A major clue came from mice genetically engineered to lack rods and cones, which are therefore blind. Just as in their sighted counterparts, light can reset the circadian clock of these animals and suppress the expression of melatonin1, a hormone produced by the brain at night that regulates the sleep–wake cycle. Similarly, some blind people also have normal sleep–wake cycles2. “We knew it existed before we knew where or what it was,” says Steven Lockley, a chronobiologist at Harvard Medical School in Boston, Massachusetts.

In 2001, Brainard’s team and researchers from another laboratory at the University of Surrey, UK, reported independently that melatonin suppression is strongest in people who are exposed to light with a wavelength of 446–477 nanometres, which corresponds to blue on the visible-light spectrum. This suggested that a receptor tuned into this light regulates the circadian clock3,4. One such receptor, the protein melanopsin, had been linked to circadian rhythm, and in 2002, researchers at Brown University in Providence, Rhode Island, showed that retinal ganglion cells containing this receptor — ipRGCs — are sensitive to light5. The biological target that enables the body’s internal clock to be reset had been identified.

Epidemiological studies in the past few decades have shown that artificial light disrupts the circadian clock, and such disruption has been linked with depression, metabolic disorders, immune and cardiovascular diseases and cancer6. Incandescent and fluorescent lighting in homes and offices does a poor job of replicating the spectrum of the Sun’s rays. A well-tuned LED could better stimulate the ipRGCs of people who are indoors during the day, helping to keep their circadian clocks on track. But replacing conventional lighting with LEDs brings its own problems: unlike incandescent and fluorescent bulbs, LED lights are often enriched with blue wavelengths that disrupt sleep when used at night. The light that LEDs produce is also more intense, meaning that they pack a double punch. “Intensity is as important as wavelength,” Lucas explains. “A bright, yellow light may have just as much melanopsin activation as a dim, blue light.”

Smarter lighting

Much of the biology that underlies ipRGCs is still being explored — for example, these photoreceptors were thought to play no part in vision, but are now known to interact with rods and cones. But researchers’ recommendations for creating a healthy lit environment are nonetheless straightforward: people should seek out bright light and blue light during the day, and minimize exposure to both at night. “I think we know enough now that we could change lighting practice for the benefit of everybody in society,” says Mark Rea, a cognitive scientist at the Lighting Research Center at Rensselaer Polytechnic Institute in Troy, New York.

Chronobiologist Steven Lockley’s laboratory at Harvard Medical School in Boston, Massachusetts, explores how the eye detects light to reset the body’s circadian clock.Credit: Magnus Wennman

A handful of light-based health interventions have already emerged. Light boxes that emit an intense blue light have been shown to help people with a form of depression called seasonal affective disorder; many mobile devices now include features to reduce the emission of blue light in the evening; and glasses that filter out such wavelengths are available. In collaboration with partners in industry, researchers are also exploring ways to make room lighting in offices, hospitals and living spaces less detrimental to health.

Rea and his colleague Mariana Figueiro, who leads the Lighting Research Center at Rensselaer, are investigating the effects of lighting interventions on older people with Alzheimer’s disease and related forms of dementia7. Because less light reaches the retina with age, a greater intensity of light is needed for photoreceptor activation, says Figueiro. Simply boosting the amount of blue light during the day helps to regulate people’s sleep–wake cycles, which are often disturbed in people with dementia. But achieving this might not always be practical. “Nobody really wants to eat their eggs under blue light — everyone looks pale and awful,” Figueiro says. “When you go into the field, you have to take that into account.” Meanwhile, Lucas and his team are using projectors to test a new type of computer or television display in which the output can be modified to reduce its ability to stimulate ipRGCs8. Conventional displays produce images by combining three colours of light — red, green and blue. Rather than draining the blue wavelengths from images, the researchers used optical filters to tweak the output of two projectors, replacing blue with violet and cyan. A fifth colour, yellow, was also used to afford the researchers greater control. The combined projections were able to produce images that were less effective at stimulating melanopsin in ipRGCs, yet had comparable colour and brightness. Volunteers were unable to tell whether images they viewed were produced by the modified displays. However, they reported feeling more sleepy and produced more melatonin in their saliva when they watched films in the evenings using the less stimulating setting.

The two types of display rely on metamerism, a phenomenon by which combinations of light that look the same actually differ in their spectral make-up, says Lucas. Each combination, or metamer, affects cones similarly, but ipRGCs differently. Lucas’s collaborator Christian Cajochen, a chronobiologist at the University of Basel in Switzerland, is planning to test the effects of such metamers on cognitive performance, mood and sleep in a study involving up to 200 office workers.

In principle, metamers could be incorporated into room lighting, enabling people to regulate the timing and strength of ipRGC stimulation indoors. But lighting a room can be tricky, explains Manuel Spitschan, a neuroscientist at the University of Oxford, UK, who uses metamers to study how light affects visual function, behaviour and brain activity, because surfaces can reflect light in many ways. So Spitschan is using computer modelling to predict how objects in a room would look when lit by metamers.

Given the effects of artificial lighting on the body, many researchers are pushing for guidelines on lighting design that take into account its effect on ipRGCs, as well as how it can make it easier to see. Last year, a group of researchers led by Schlangen worked with the International Commission on Illumination, a non-profit organization in Vienna, to create a measurement standard. It should help to translate peer-reviewed findings into quantitative guidance for lighting design.

More from Nature Outlooks

Installation of the dynamic lighting system on the ISS is expected to be completed later this year. It has been designed to offer three settings: one that produces a bright, white light for use during working hours; another that makes a dim light depleted in blue wavelengths to help prepare astronauts for sleep in the ‘evening’; and a higher-intensity light enriched in blue wavelengths that will be used to help boost alertness when required and to reset the circadian clock after working at night or to fix disrupted sleep–wake cycles. Brainard and Lockley, who are leading the project, have already assessed the system’s effects on astronauts’ sleep, melatonin levels, work performance and vision on Earth. Now, astronauts will run the same tests in space to determine whether such lighting can override the effects of experiencing 16 sunrises a day.

Demonstrating that it is possible to modulate the extreme circadian disruption associated with living in space will help to build the foundation of a smart-lighting future, the pair says. “We’re fortunate that the neuroscience and the technology of LEDs has evolved at the same time,” says Lockley. “It’s only going to get more interesting.