For many of us, night has become day. We work, travel, shop, exercise and socialise in hours that used to be reserved for relaxation and sleep. Time is a limited resource and, to make full use of it, the night has been illuminated and occupied. Even when we do sleep, street lamps and security lights pierce the darkness.
But our freedom from the natural constraints of day and night may have come at a price. According to a growing band of scientists and doctors, many of us are no longer getting enough darkness in our lives. The theory is based on a simple premise. Our biological rhythms evolved in a time before artificial light, to take advantage of both bright days and dark nights. By succumbing to the temptations of 24-hour living, and ignoring or reducing our natural dark time, we could be putting our health at risk.
"A number of health and environmental problems are due to a loss of darkness," says Dr David Crawford, executive director of the International Dark-Sky Association, a group that campaigns against light pollution. "And it will get worse as we creep - or rush - to a 24/7 world. All of life, all of it, has evolved with a day/night cycle - the circadian rhythm. It's essential to good health. Many studies are now showing that those who go without a true day/night cycle are adversely impacting their immune systems, and that's not good."
It's not good, but it's becoming the norm. More than 20% of the working population now work at least some of the time outside the 7am-7pm day. Global travel, the internet, job insecurity, 24-hour shopping and TV, and - coming soon - late-night pubs and bars, all help to push back the boundaries of the active day. To remain active at night we need light, with the result that the natural circadian cycle of day and night, light and dark, is becoming perilously unbalanced. We are creating a conflict between what we want to do, and what our internal timekeeper - the body clock - prepares us for.
"Our biological clock has been likened to the conductor of an orchestra, with the multiple rhythms of the body representing the various sections of that orchestra," says Russell Foster, professor of molecular neuroscience at Imperial College, London, which later this month hosts the first international sleep conference. "The body clock adapts us for the varying demands of activity and rest. It ensures our internal synchronicity: that our various internal systems - temperature, alertness, blood pressure and so on - are working together. And the body clock sets itself using the light/dark cycle. By moving to 24-hour living, and reducing or ignoring the dark bit, we are effectively throwing away the advantages of millions of years of evolution."
The effects of screwing up our body clocks are most readily observable in the growing army of night workers. Studies suggest that, even after years of night shifts, many workers never adjust to a regime that pitches them against our basic and hard-wired biology. Instead, they head wearily home to bed just as the morning light is prompting their body clocks to prepare for activity, and back again when the gathering dusk tells them to prepare for rest.
Once at work, overriding the craving for dark and sleep comes at a price. "They activate the 'fight or flight' stress mechanism," says Foster, "and we know that stress in turn can suppress the immune system." Bright lights, caffeine and nicotine artificially maintain stimulation. Perhaps unsurprisingly, studies show that nightshift workers are at increased risk of a range of health problems, from stress, constipation and stomach ulcers to depression, heart disease and cancer. For example, a 2001 study in Seattle, based on interviews with 800 women, found that females who worked the graveyard shift could face a 60% increase in the risk of breast cancer.
There is another theory that tries to explain the high incidence of breast and colo-rectal cancers in shift workers, however. Melatonin is called "the Dracula hormone" because it always comes out at night. But its production can be severely reduced by bright artificial light. The effects of melatonin on health are not properly understood, but a number of scientists, particularly in America, are connecting low levels of melatonin with high levels of certain cancers in nightshift workers. One study presented to the American Association for Cancer Research found that melatonin can slow tumour growth by up to 70% in mice infected with human breast cancer cells. When the mice were subjected to constant light, cancer growth rocketed.
Some have taken it further. George Brainard, a neurologist at Thomas Jefferson University in Pennsylvania, has recommended that we all exercise a "prudent avoidance" of light at night to ensure normal levels of melatonin whether we work night shifts or not. Brainard's research has shown that the human body clock can be affected by light of short wavelength, which is more prevalent in artificial light used at night. It also showed that melatonin production was reduced by just this sort of short wavelength light. Until recently, it was believed that only daylight was strong enough to influence our internal systems.
On the back of this, another researcher has advised parents not to let children sleep with the light on because of a potential - though unproven - connection between artificial light at night and childhood leukaemia. The incidence of leukaemia in children under five rose by 50% in the second half of the 20th century, leading some scientists to point the finger at our increased reliance on artificial light. "I would not myself use a nightlight in a kid's bedroom unless there is a reason for it for safety," said Richard Stevens, a cancer epidemiologist at the University of Connecticut, after a conference on childhood leukaemia in London last autumn. "There is interesting evidence about melatonin having properties that would lead to reduction in cancer risks, so the possibility that this might be related to childhood leukaemia is important."
Foster, on the other hand, believes that many of these claims are "overselling" the evidence of melatonin's anti-cancer properties. "It's true that melatonin is suppressed by light, but in reality we don't really know what effects it has on health. Its anti-cancer properties are a long way from being established as fact," he says. "My opinion is that the suppression of the immune system is much more likely to explain cancer rates in shift workers."
Nevertheless, most scientists agree that our rush to turn night into day must be having some effect on our health, even if we don't work the graveyard shift. "We now know that we are sensitive to the sort of light levels we readily expose ourselves to in the evening," says Dr Derk-Jan Dijk, director of the Sleep Research Centre at Surrey University. "We're not quite sure what the impact is, but we know that even ordinary room light can have an effect on our physiologies."
A television that flickers all night in a child's bedroom ... street lighting that spills through curtains ... "There's not enough research on these things at the moment," says Dijk, "but it's certainly a concern. Ideally, we should all be sleeping in darkened rooms. And don't forget, in the natural situation, in which we evolved, dark really did mean dark."
For the general population, the most pressing problem stemming from ubiquitous artificial lighting and 24-hour living is sleep deprivation. The absence of true, continuous darkness could be affecting the quality of our sleep. It's certainly affecting the quantity. We are stretching the boundaries of day at one end, without being able to stretch the boundaries of night at the other.
"I think the critical issue is that sleep has been greatly delayed by our invasion of the night," says Foster. "So we try to manipulate our body clocks with stimulants and sedatives. Caffeine and nicotine keep us awake. Alcohol and hypnotics counteract them when we want to sleep. It's a worrying cycle and the 24-hour society promotes it."
Humans have known for a long time that banishing the dark from our lives has a powerful effect. "Don't forget," reminds Dijk, "continuous light has long been used as a method of torture."
· The first Fatigue, Sleep and Biological Clocks International Conference is to be held at Imperial College, London, on March 31 and April 1.