Darkness is important for optimum reproductive health in women, and for protecting the developing foetus, said study researcher Russel J Reiter, a professor of cellular biology at the University of Texas Health Science Center in San Antonio.
In a review of studies published in the journal Fertility and Sterility, Reiter and his colleagues evaluated previously published research, and summarised the role of melatonin levels and circadian rhythms on successful reproduction in females.
The evidence showed that every time the light at night was turned on, it turned down the production of melatonin, 'Live Science' reported.
Melatonin, a hormone secreted by the pineal gland in the brain in response to darkness, is important when women are trying to conceive, because it protects their eggs from oxidative stress, Reiter said.
"If women are trying to get pregnant, maintain at least eight hours of a dark period at night. The light-dark cycle should be regular from one day to the next; otherwise, a woman's biological clock is confused," he said.
Eight hours of darkness every night is also optimal during pregnancy, and ideally, there should be no interruption of nighttime darkness with light, especially during the last trimester of a pregnancy, Reiter said.
Turning on the light at night suppresses melatonin production in women, and means the foetal brain may not get the proper amount of melatonin to regulate the function of its biological clock, he said.