You know the story: your teen arrives home from school looking suspiciously green around the gills. Did he have a bad day? Is he feeling sick? No, but there's a huge test tomorrow, and he's going to have to study 'til sunup to pass the class.

Although everyone knows that pulling an all-nighter is no way to go, many see no alternatives to the occasional late-night cram session. The material has got the get in there somehow, right? But according to William Kohler, M.D., a pediatric sleep specialist at the Florida Sleep Institute, all-nighters may be doing more harm than good. “The person that crash-studies is going to perform worse than if he had slept properly the night before the exam,” he explains. “It decreases the ability to concentrate and perform your best the next day.”

And it's not just all-nighters on test days that can adversely affect students. Studies have shown that lack of a proper night's sleep can affect concentration in the classroom, emotional well-being, and may even be putting kids at an increased risk of becoming overweight. So, how much is enough? The American Academy of Sleep Medicine recommends 11-13 hours of shut-eye for preschoolers, 10-11 for school-aged children, and around 9 hours for adolescents.

That's all well and good, but what about your stressed-out teen, who's reaching for the coffee-pot at 10pm? Sure, coffee will keep him up, but will it really help him focus? Maybe a little, says a new study, but not as much as a nap. In the study, Patricia Sagaspe, Ph.D., studied the effects of both coffee intake and napping on late-night drivers. The results? While both strategies helped tired drivers stay within the lane lines, napping was a significantly better countermeasure in younger participants. Translation: while coffee may keep them up, kids will probably perform better after a nap. “Power napping for 20-30 minutes can be very valuable,” says Kohler. “A short nap can be very beneficial and refreshing.” But nappers beware: snooze too long, and you'll wake up feeling more tired than ever.

So what's the solution for sleep-deprived kids? According to Kohler, the key to getting enough sleep is to make it routine. He recommends going to bed and waking up at the same time each day, avoiding caffeine in the evening, and getting plenty of exercise (but not right before bedtime). As for your tired teen? Tell him to hit the hay for half an hour. And next time, make sure that studying gets done before the last minute, so you and your teen can sleep in peace.