If asked to make a list of your basic needs for survival, what would come on top? Food and water, perhaps? Or a warm shelter? Darkness is an unlikely contender – the closest you might come is sleep.
According to the latest science, that could be an oversight. Cheng Chi Lee, who studies circadian rhythms at the University of Texas Medical School, says there is now a growing body of evidence suggesting that we should revel in darkness because it can have surprising effects on health and behaviour.
One remarkable experiment in 2013 showed that a period of complete darkness could help restore bad eyesight. Kittens were given amblyopia, a condition better known as lazy eye, where the vision in one eye is impaired because of poor connections to the brain. It affects about 4% of the human population and if left untreated can lead to blindness. The team, from Dalhousie University in Canada, deprived seven kittens of visual input to one eye and measured their vision in both the good and bad eye. They were then kept in complete darkness for 10 days, after which vision in the damaged eye made a startling recovery. The researchers suspect that darkness somehow resets the vision system to an earlier stage of development when it is more adaptable.
Cancer treatment, too, could benefit from total darkness. A recent study compared the effect of the drug tamoxifen on cancer cells in mice. In one arm of the study, the mice were kept in cycles of 12 hours of light followed by 12 hours of complete darkness. In another, the dark stage of the experiment was replaced with 12 hours of very dim light – roughly equivalent to the light that might creep in under a hospital door. Even in such low levels of light, the cancer cells became resistant to the drug. Although the experiment was in mice, the team from Tulane University in New Orleans believe this could have important implications for the way cancer patients receive their treatment.
Lee’s own work has shown that plunging mice into darkness seems to trigger a change in fat metabolism, switching from burning glucose to fat. The mechanisms behind the switch helps explain what happens during mammalian hibernation, but might also have relevance in humans, not least because the genes that regulate circadian functions are all highly conserved between human and other mammals. “There are primate species that can hibernate, so I believe the biological processes are most likely conserved in humans,” he says. The switch might make a good target for treating obesity and type-2 diabetes.