Ever saw a sleepy student get all cranky and moody while in school? It's not the lack of sleep that caused that - it might have been caused by staying up late at night, and then feeling sleepy by day. That's what a new study has found.

According to a study published in Pediatrics, researchers looked into the effects of various sleep factors on a student's self-regulation, or their ability to control their emotions and manage themselves. They surveyed more than 2,010 students between seventh to 12th grade in 19 schools in Virginia.

The students answered questions about their sleep duration, preference for morning or evening (or chronotype), daytime sleepiness, their cognitive abilities, and the ability to manage their emotions and behavior.

What they found out was quite interesting: no matter how much sleep a student gets, his self-regulation is negatively affected by being a "night owl," or going to bed late in the evenings, and then feeling sleepy during the day.

"The take-home message here is that it's not just how much you sleep, it's when you sleep," Dr. Judith Owens, director of sleep medicine at Boston's Children's Hospital and lead researcher, said, reports NPR.

Owens adds that in order to help solve this problem, schools should start their classes later.

"Getting these kids enough sleep and appropriately timed sleep is necessary for optimal self-regulation," she said. "If you don't have enough and appropriately timed sleep, then you're going to compromise your ability to have these kinds of skills."

In a commentary that accompanies the study, Duke University sleep researcher Dr. Sujay Kansagra says that starting school late will greatly help students.

"Starting schools late helps cater to where adolescents would function at their best," Kansagra said.

Study authors note, however, that they could not pinpoint a certain cause, and recommends that teens work with clinicians especially if they are still having problems with self-regulation even if they normally get the recommended amount of sleep, which is eight to 10 hours per night, according to the American Academy of Sleep Medicine.