For women who want to conceive, stop staying up late at night as every time you turn on the light, it slows down the production of the fertility hormone.
"Darkness is important for optimum reproductive health in women and for protecting the developing foetus," said Russel J Reiter, a professor of cellular biology at University of Texas' health science centre in San Antonio.
Melatonin, a hormone secreted by the pineal gland in the brain in response to darkness, is important when women are trying to conceive.
"Melatonin has strong antioxidant properties that shield the egg from free-radical damage, especially when women ovulate," Reiter added.
If women are trying to get pregnant, maintain at least eight hours of a dark period at night, researchers said. Staying in darkness has nothing to do with sleep.
"It is the darkness that is needed for the brain to produce melatonin," Reiter was quoted as saying in media reports.
The study appeared in the journal Fertility and Sterility.