Sleeping in a room with too much light can lead to depression, psychologists claim.
Leaving the television on while you sleep can be enough to trigger the effect, say scientists.
Lack of darkness during sleeping hours can cause changes to the brain and depressive symptoms, according to animal studies.
Researchers believe staying up late to watch TV or go online might have the same impact.
But, tests carried out on hamsters have shown that the harmful effects could be reversed by returning to natural light, the Daily star reported.
“The good news is that people who stay up late in front of the television or computer may be able to undo some of the harmful effects just by going back to a regular light-dark cycle and minimising their exposure to artificial light at night,” Tracy Bedrosian, of Ohio State University, said.