Electric lights in laptop computers, smartphones and tablets are negatively affecting people’s sleep, an expert has said.
Artificial lights disrupt the body's natural rhythm, affect chemicals in the brain and drive people to use stimulants like caffeine to stay awake longer, according to Harvard academic Professor Charles Czeisler.
Writing today in the journal Nature, the professor of sleep medicine at Harvard Medical School called for research to help develop "behavioural and technical" ways of counteracting the ill effects of artificial light on modern sleeping patterns.
The decline in the number of hours slept per night is affecting public health, including a greater risk of obesity, diabetes, heart disease, depression and stroke in adults and concentration problems in children, he said.
While all electric light affected circadian rhythms – the natural body clock – and sleep, night-time exposure to LED lights like those in phones and computers was "typically more disruptive" than standard electric light bulbs, he said.
"There are many reasons why people get insufficient sleep in our 24/7 society, from early starts at work or school, or long commutes, to caffeine-rich food and drink," he wrote.
"But the precipitating factor is an often unappreciated, technological breakthrough: the electric light. Without it, few people would use caffeine to stay awake at night. And light affects our circadian rhythms more powerfully than any drug."
Between 1950 and 2000, the average person in the UK increased their use of artificial light sources by four times, with a parallel rise in sleep deficiency, Prof Czeisler said.
Artificial light inhibits sleep-promoting neurons in the brain and the nightly release of the hormone melatonin, which aids sleep, while activating neurons that boost alertness, he said.
It fools the brain into delaying its "second wind", which kicked in during the afternoon to see people through to sunset before electric light was invented, until much later in the day.
He said that as a result of modern technology "many people are still checking email, doing homework or watching TV at midnight, with hardly a clue that it is the middle of the solar night".
In a study, 30 per cent of all working adults in the United States and 44 per cent of night workers reported getting less than six hours sleep a night on average. Fifty years ago less than 3 per cent of the US adult population slept so little.
And around the world, children are sleeping 1.2 hours less on school nights than a century ago, he said.
"Technology has effectively decoupled us from the natural 24-hour day to which our bodies evolved, driving us to go to bed later," he said. "And we use caffeine in the morning to rise as early as we ever did, putting the squeeze on sleep."