Experts say too-early school start times are one big factor, reports Healthday.

“I don’t believe there’s one culprit,” said the study’s lead author, Charles Basch, a professor of health and education at the Teacher’s College at Columbia University in New York City.

“For some children it’s too much homework, for some it’s health problems like asthma,” he explained. “For others it may be anxiety or depression, or the prescription medications they are taking for such conditions. Recreational drugs can be a factor, as can having electronics in the bedroom.”