The dangers of light at night

The workhorse of this system is the light-sensitive hormone melatonin,which is produced by the body every evening and during the night. Melatonin promotes sleep and alerts a variety of biological processes to the approximate hour of the day.

Light hitting the retina suppresses the production of melatonin—and there lies the rub. In this modern world,our eyes are flooded with light well after dusk,contrary to our evolutionary programming. Scientists are just beginning to understand the potential health consequences. The disruption of circadian cycles may not just be shortchanging our sleep,they have found,but also contributing to a host of diseases.

“Light works as if it’s a drug,except it’s not a drug at all,” said George Brainard,a neurologist at Thomas Jefferson University in Philadelphia and one of the first researchers to study light’s effects on the body’s hormones and circadian rhythms.

Any sort of light can suppress melatonin,but recent experiments have raised novel questions about one type in particular: the blue wavelengths produced by many kinds of energy-efficient light bulbs and electronic gadgets. Brainard and other researchers have found that light composed of blue wavelengths slows the release of melatonin with particular effectiveness.

Until recently,though,few studies had directly examined how blue-emitting electronics might affect the brain. So scientists at the University of Basel in Switzerland tried a simple experiment: They asked 13 men to sit before a computer each evening for two weeks before going to bed. During one week,for five hours every night,the volunteers sat before an old-style fluorescent monitor emitting light composed of several colours from the visible spectrum,though very little blue. Another week,the men sat at screens backlighted by light-emitting diodes,or LEDs. This screen was twice as blue.

“To our surprise,we saw huge differences,” said Christian Cajochen,who heads the Center for Chronobiology at the University of Basel. Melatonin levels in volunteers watching the LED screens took longer to rise at night,compared with when the participants were watching the fluorescent screens,and the deficit persisted throughout the evening.

The subjects also scored higher on tests of memory and cognition after exposure to blue light,Cajochen and his team reported in the May issue of The Journal of Applied Physiology. While men were able to recall pairs of words flashed across the fluorescent screen about half the time,some scores rose to almost 70 per cent when they stared at the LED monitors.

The finding adds to a series of others suggesting,though certainly not proving,that exposure to blue light may keep us more awake and alert,partly by suppressing production of melatonin.

What do these findings mean to everyday life? Some experts believe that any kind of light too late into the evening could have broad health effects,independent of any effect on sleep. For example,a report published last year in the journal PNAS found that mice exposed to light at night gained more weight than those housed in normal light,even though both groups consumed the same number of calories.

Light at night has been examined as a contributor to breast cancer for two decades. While there is still no consensus,enough laboratory and epidemiological studies have supported the idea that in 2007,the World Health Organization declared shift work a probable carcinogen.

Researchers like Brainard hope the science may lead to a new generation of lights and screens designed with wavelengths that adjust according to the hour of the day.

Among those interested are officials at NASA,who have approached the neurologist about designing light on the International Space Station in a way that promotes alertness during waking hours and encourages sleep during times of rest.

“I think we’re on the verge of a lighting revolution,” said Brainard. If the hormone-sparing lights can be made to work during spaceflight,he said,“people will use it here on the ground.”