Bright street lighting and office blocks that remain lit all night could be affecting our mental health, scientists have warned.
Researchers said too much light at night can be linked to depression. Those living in cities have long complained that fluorescent street lights affect their ability to sleep and can alter their mood.
Now psychologists have confirmed that being unable to escape to the dark can have a harmful effect on someone's personality.
In tests on mice, the study authors found that those kept in a lit room 24 hours a day showed more depressive symptoms than those that had a normal light-dark cycle. Mice that lived in constant light, but could escape into a dark tube when they wanted showed less evidence of depressive symptoms than those who had no escape.
Co-author Professor Randy Nelson said the results suggested more attention needed to be focused on how artificial lighting affects emotional health in people.
He said: 'The increasing rate of depressive disorders in humans corresponds with the increasing use of light at night in modern society. Many people are now exposed to unnatural light cycles, and that may have real consequences for our health.'
The study, published in the journal Behavioural Brain Research, involved 24 male laboratory mice.