Like that of adults, children’s screen time has gone up over the years, and rose significantly during the COVID-19 pandemic. Kids between ages 8 and 12 spend between four and six hours using or watching screens per day, and teens spend up to nine hours, according to the American Academy of Child and Adolescent Psychiatry. This is dramatically more than the one hour of daily screen time pediatricians recommend for children ages 2 through 5.

Blue light, or high-energy visible light, is produced by the sun but also by our screens, which has left experts concerned over how exposure to blue light through devices is affecting our health. Blue light exposure has been previously linked to disruptions in the circadian rhythm and people’s sleep cycles, according to Harvard Medical School.

In a study of rats, higher levels of blue light exposure were associated with an earlier onset of puberty for females, according to new research presented on September 16 at the 60th Annual European Society for Paediatric Endocrinology Meeting. The study also noted the exposure was associated with lower levels of melatonin, higher levels of some reproductive hormones, and changes in the physical makeup of the ovaries.

In the study, different groups of rats were exposed to a normal light amount, six hours of blue light, and 12 hours of blue light per day. The rats exposed to the longest duration of blue light experienced puberty the earliest. The rats exposed to blue light also had reduced levels of melatonin, which is the hormone that affects sleep, and the rats exposed to blue light for the longest had cell damage and inflammation in their ovaries. Melatonin levels are at their highest during prepuberty, compared with during puberty, which “which is an inhibitory factor in the onset of puberty,” Dr. Aylin Kilinç Uğurlu, the study’s author, tells Fortune.

“Blue light is the light that suppresses melatonin levels the most,” Uğurlu says. “Exposure to blue light decreases melatonin levels and causes early onset of puberty [in rats] by decreasing its inhibitory effect at the onset of puberty.”

During the pandemic, physicians reported more girls experiencing early puberty, according to the Washington Post, and the reasons why are still largely unknown. A study carried out in Turkey found that the number of girls diagnosed with early puberty owing to unknown reasons more than doubled in the first year of the pandemic compared with the previous three years. Early onset of puberty has been linked to higher levels of mental health issues, including depression and social anxiety, according to the American Psychological Association.

While the results in rats cannot be generalized for children, Uğurlu says, the “hormonal mechanism,” including the ovulation process in, before, and after puberty, is the same in rats and humans.

The findings may lead to blue light being categorized as a risk factor for early onset of puberty, Uğurlu says, according to the press alert.

The study, at its core, shows that blue light exposure and its association to early onset puberty needs to be examined more thoroughly.

“In the coming years, as the place of mobile electronic devices in our lives will increase gradually, blue light exposure in childhood will also increase,” says Uğurlu. “We think that the use of mobile devices, known as the inevitable blue light source, should be prevented in the childhood age group, especially in the evening hours, and we should limit the usage times.”