Light has a dark side Anne-Sophie Bost/PhotoAlto/Getty

Too much light is bad for your health. So suggests research in mice, which found that six months of continuous lighting led to a loss of strength and bone mass, and signs of increased inflammation. The findings are worrying for people who experience prolonged light exposure – such as shift-workers and hospital patients – but some of the effects seem to be reversible.

The experiment involved 134 mice, which experienced no dark for half a year. By the end, the mice had lost about half their strength compared with controls, as measured by grip endurance tests and their ability to cling to bars, while the signals of their internal body clocks were weakened.

Their bones were affected too. The bulbous, spongy parts of their bones that are responsible for bearing most weight lost a third of their volume, and became 10 per cent thinner – just as in the early stages of osteoporosis.


There were also signs of increased inflammation, such as a rise in the number of neutrophil white blood cells – usually associated with stress or infection.

Fast recovery

These effects may be connected to disruption of the animals’ internal clocks. Under normal conditions, there is a clear difference between the signals given by clock neurons in the brain during the day and the night. When exposed to constant light, the difference in the strength of these signals was reduced by 70 per cent.

The results support evidence from studies that have suggested that prolonged light exposure in people can affect health. For example, women who experience longer periods of light are more likely to fracture their bones, and such conditions also seem to be linked to an increased risk of cancer and metabolic diseases.

However, the team, which was led by Johanna Meijer of Leiden University Medical Center in the Netherlands, found that the animals – and their disrupted circadian rhythms – recovered when their dark night-times were restored. “The clock recovered near instantaneously,” says Meijer, while muscles and bones recovered their strength in about two weeks.

This is good news, says Meijer, because it means that people experiencing prolonged light exposure – such as patients in intensive care or premature babies in neonatal units – can be helped by introducing periods of darkness. This could need extra measures, as even when someone has their eyes closed, light can pass through their eyelids and confuse their internal clock. “You could, for example, cover their eyes at night with masks like those available to plane passengers,” says Meijer. “It shouldn’t be too hard to apply this knowledge in the real world.”

Disrupted sleep

“This is excellent work, building on mounting empirical evidence and decades of anecdotal reports from shift workers, jet-setters and all-nighter college students that circadian disruption by light can result in significant health effects,” says Ilia Karatsoreos of Washington State University in Pullman.

Steven Lockley of Harvard Medical School says the experiment’s effects may not be solely down to the effects of constant light exposure on the internal clocks of the mice, and could be due to altered sleep instead.

Although the effects in mice are worrying, they may not directly apply to humans, because mice are naturally nocturnal so may suffer more from a lack of darkness. “Six months is about a quarter of a mouse’s lifespan, and light can have very different effects on a nocturnal species,” says Karatsoreos.

Journal reference: Current Biology, DOI: 10.1016/j.cub.2016.05.038

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