A runner passes the Eiffel Tower during sunrise in Paris, France. [Photo/Agencies]

In October, the Nobel Prize in physiology or medicine went to three American academics, Jeffrey Hall, Michael Rosbash and Michael Young, for their work on circadian clocks. They've established that plants, animals and humans naturally adapt their biological rhythm－the cycle of rest and activity－so that it's synchronized with the Earth's revolutions, with the cycles of light and darkness.

Their work draws attention to recent changes in the way we live. In the past 100 years, humans have ignored natural light-dark cycles, and relied on electric lighting, particularly screen lights to keep us awake when it's dark outside.

We're just beginning to understand the consequences of this dramatic change in lifestyle. Links have been suggested between a malfunctioning of our circadian clock and both physical and psychological illnesses.

Ellen Stothard and colleagues at the University of Colorado have shown that when individuals rely on artificial lighting to keep them awake, their circadian clocks are overridden and start to run "late". This is linked with higher rates of obesity, poor school performance and mood disorders such as depression.

These are only associations. However, in a paper presented to the Proceedings of the National Academy of Sciences in 2011, Ilia Karatsoreos and colleagues at Rockefeller University reported that when the connection between rest, activity cycles and environmental light, and dark cycles in mice was deliberately broken, many became obese. There was a decrease in the neurons in the prefrontal cortex, the area associated with emotional control, and cognitive functioning became more rigid.

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