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."