Sleeping with the light on could leave you feeling low the next day, scientists have warned.
They say that a night-light – however dim – may affect the structure of the brain, raising the odds of depression.
The eerie glow emitted by a TV or the seemingly reassuring presence of a night-light could be enough to impact on mental health.
It is the latest in a long line of warnings about the potential dangers of disrupting the body’s natural sleep-wake cycle.
There are concerns that shift workers are at higher risk of breast cancer and, only last month, a study linked night-time light with weight gain.
With complete darkness being hard to achieve in the modern world, experts say the findings could have serious implications for health.
In the latest study, presented at an American conference, researchers from Ohio State University looked at the effect of exposing rodents to dim light in the eight hours or so they would usually be asleep.
At the equivalent of having a television on in a darkened room, the lamp used was not bright, but it was enough to affect the animals’ behaviour, the Society for Neuroscience’s annual conference heard.
One depression test measured how much sugared water the creatures drank, with a lack of interest in the sweet treat signalling a change in mood.
Overall, those exposed to the dim light at night showed more symptoms of depression compared with the hamsters in the standard light-dark cycle.
Further tests showed differences in the structure of a brain region called the hippocampus.
Researcher Tracy Bedrosian said: “The hippocampus plays a key role in depressive disorders, so finding changes there is significant. Even dim light at night is sufficient to provoke depressive behaviour.”
Neuroscientist Dr. Randy Nelson said: “The light was a very low level. Something that most people could easily encounter every night.”