Modern habits such as working late into the night and ‘unwinding’ by staring at a phone screen could increase the risk of depression, a study has found.
Researchers found that exposure to blue light during the night led to mice becoming less active and eating less, after exposure to just two hours of blue light.
The researchers believe that light sensitive cells in the retina affect areas in the brain linked to negative emotions.
Corresponding author Dr Huan Zhao, of Hefei University, said: "Besides generating vision, light modulates various physiological functions, including mood.
"While light therapy applied in the daytime is known to have anti-depressive properties, excessive light exposure at night has been reportedly associated with depressive symptoms."
The findings suggest artificial light causes depression in both nocturnal and day living animals such as mice and humans respectively, irrespective of the body clock.
The researchers believe that the brain ‘circuit’ may have evolved to protect us, but is now causing depression in humans in our brightly lit, 24-hour world.
Dr Zhao said: "Conceivably, this circuit might prompt animals to shy from unnecessary light exposure during the subjective night which might help the animal to avoid predators or to prevent disturbing their biological rhythms - especially during transition periods such as dusk or dawn.
"However, unlike any other time in our evolutionary history, exposure to light-at-night is increasingly inevitable in the post-industrial era.
"Consequently, this pathway, which has been adapted to serve protective purposes, might be 'hijacked' to fuel negative mood in humans."
At any one time more than 300 million people have depression – one in 25 of the world's population. The breakthrough offers hope of better ways to combat the epidemic.
Dr Zhao said: "Exposure to excessive light-at-night – either from 'sky-glow' or the use of electronic devices such as mobile phones or personal computers before bedtime – has been associated with a greater risk for depressive symptoms.
"These contradicting effects of light during different circadian phases suggest the timing of light exposure could be a critical factor.
"Insights into the neural mechanism behind this apparent paradox are fundamental to our understanding of mood regulation by light."