Staying up late to study more is a no-brainer. Unfortunately, it's the wrong kind of no-brainer.

Trading sleep for extra studying doesn't work. It leaves your brain fuddled the next day and lowers grades on tests and homework, as well as general comprehension. That's what a close look at high school students' own diaries shows, diaries that kept track of the students' sleep and studying habits and how they affected their performance in school.

The message is that if you need to do some extra studying, do it with time you'd ordinarily spend on other activities, like watching TV. Don't let it cut into your sleep.

More studying meant less sleep and poorer performance the next day, whether or not there was a test.

UCLA researchers tracked 535 Los Angeles students throughout their high school years. In the 9th, 10th and 12th grades, each student filled out a diary for two weeks during the school year. The diaries reported how long the students studied each night, how long they slept and whether they experienced two types of academic problems in school--not understanding something that was taught in class or receiving a poor grade on a test, quiz or homework.

The researchers found that on nights when students studied more, they experienced more academic problems the next day. And this appeared to be driven by getting less sleep on those nights.

This wasn't true in the 9th grade but became noticeable in the 10th grade and was even stronger in the 12th grade.

The drop-off in academic performance occurred whether or not the students had a test the next day. It wasn't just a consequence of studying for a difficult test. More studying meant less sleep and poorer performance the next day, whether or not there was a test.

Overall, students reported studying for an average of a little over an hour a night throughout their high school years. But their nightly sleep decreased, averaging 41 less minutes of sleep per night in the 12th grade than in 9th grade. Sleep fell from 7.6 to 7.4 to 6.9 hours nightly as students progressed from 9th to 10th to 12th grade. Nine hours are recommended.

So students start out high school getting less sleep than they should and the problem worsens with each succeeding year.

One helpful solution can be to start school later in the day. But that's a decision that's usually out of the students' hands.

The study authors suggest maintaining a consistent study schedule across the entire school year, using school time more efficiently (studying during homeroom and open periods) and shifting time from less essential activities into studying are the best ways for students to avoid the need to steal study time from sack time.

An article on the study was published online by Child Development on August 20, 2012 and will also appear in a future print issue of the journal.