For many, the harsh, bright glare of fluorescent street lights or office blocks which remain lit overnight has led to sleepless nights and subsequent bad moods. But now psychologists from Ohio State University in the US, say that being unable to escape to the dark can affect personality, a person’s health and could lead to depression.
“The ability to escape light seemed to quell the depressive effects,” said Laura Fonken, the study’s lead author.
“But constant light with no chance of escape increased depressive symptoms.”
Miss Fonken said the results provide additional evidence that the use of artificial light at night may have harmful effects on health.
“This is important for people who work night shifts, and for children and others who watch TV late into the night, disrupting their usual light-dark cycle,” she said.
The researchers also said many hospital wards are brightly lit all night, which may add to the problem of in-patients.
Co-author Prof Randy Nelson, a neuroscience expert, 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 tests on 24 male laboratory mice, where it found those kept in a lighted room 24 hours a day showed more depressive symptoms than those that had a normal light-dark cycle.
But mice that lived in constant light, but could escape into a dark opaque tube when they wanted showed less evidence of depressive symptoms than did mice that had 24-hour light, but only a clear tube in their housing.
Half were housed in light for 16 hours a day and darkness for eight hours, while the other half had 24 hours of light.
Half of each group had dark tubes in their units that let them escape the light when they chose. The other half had similar tubes that were clear and let light in.
After three weeks, the mice began a series of tests that are used to measure depression and anxiety in animals.
Several of these tests are the same ones used by pharmaceutical companies to test anti-depressive and anti-anxiety drugs in animals before they are used in humans.
One depression test measured how much sugar water the mice drank. Mice generally like the drink, but those with depressive-like symptoms will not drink as much, as they do not get as much pleasure from activities they usually enjoy.
In all the tests, mice housed in constant light with no chance to escape showed more depressive-like symptoms than those mice with normal light-dark cycles.
In some tests, mice that had tubes where they could escape the constant light showed no more depressive-like symptoms than did mice housed in normal light-dark cycles.
Unexpectedly, the results showed that, compared to the other mice, those that were housed in constant light actually showed lower levels of anxiety and lower levels of corticosterone, a stress hormone linked to symptoms of anxiety.
That was unexpected because anxiety and symptoms of depression often go together in humans.