A sleep deficit affects everything from someone's ability to pay attention in class to his or her mood. According to a National Sleep Foundation Sleep in America poll, more than 25% of high school students fall asleep in class, and experts have tied lost sleep to poorer grades. Lack of sleep also damages teens' ability to do their best in athletics.

Slowed responses and dulled concentration from lack of sleep don't just affect school or sports performance, though. More than half of teens surveyed reported that they have driven a car while drowsy over the past year and 15% said they drove drowsy at least once a week. The National Highway Safety Traffic Administration estimates that more than 100,000 accidents, 40,000 injuries, and 1,500 people are killed in the U.S. every year in crashes caused by drivers who are simply tired. Young people under the age of 25 are far more likely to be involved in drowsy driving crashes.

Lack of sleep also is linked to emotional troubles, such as feelings of sadness and depression. Sleep helps keep us physically healthy, too, by slowing the body's systems to re-energize us for everyday activities.

Even if you think you're getting enough sleep, you might not be. Here are some of the signs that you may need more sleep:

Some researchers, parents, and teachers have suggested that middle- and high-school classes begin later in the morning to accommodate teens' need for more sleep. Some schools have implemented later start times. You and your friends, parents, and teachers can lobby for later start times at your school, but in the meantime you'll have to make your own adjustments.

Here are some things that may help you to sleep better:

that it's time to sleep. Waking up at the same time every day also can help establish sleep patterns. So try to stick as closely as you can to your sleep schedule, even on weekends. Try not to go to sleep more than an hour later or wake up more than 2 to 3 hours later than you do during the week.

make it harder to fall asleep. Finish exercising at least 3 hours before bedtime. Many sleep experts believe that exercising in late afternoon may actually help a person sleep.

p.m. Nicotine is also a stimulant, so quitting smoking may help you sleep better. And drinking alcohol in the evening can make a person restless and interrupt sleep.

Staying away from bright lights (including computer screens!), as well as meditating or listening to soothing music, can help your body relax. Try to avoid TV, computers and other electronics, and using your phone (including texting) at least 1 hour before you go to bed.

studied less but got more sleep.

room that is slightly on the cool side. Close your blinds or curtains (and make sure they're heavy enough to block out light) and turn down the thermostat (pile on extra blankets or wear PJs if you're cold). Lots of noise can be a sleep turnoff, too. Use a nature sounds or white-noise machine (or app!) if you need to block out a noisy environment.

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If you're drowsy, it's hard to look and feel your best. Schedule "sleep" as an item on your agenda to help you stay creative and healthy.

Until recently, teens often got a bad rap for staying up late, oversleeping for school, and falling asleep in class. But recent studies show that adolescent sleep patterns actually differ from those of adults or kids.

Experts say that during the teen years, the body's circadian rhythm (sort of like an internal biological clock) is temporarily reset, telling a person to fall asleep later and wake up later. This change might be due to the fact that the brain hormone melatonin is produced later at night for teens than it is for kids and adults. This can make it harder for teens to fall asleep early.

These changes in the body's circadian rhythm coincide with a busy time in life. For most teens, the pressure to do well in school is more intense than when they were kids, and it's harder to get by without studying hard. And teens also have other time demands — everything from sports and other extracurricular activities to working a part-time job to save money for college.

Early start times in some schools also might play a role in lost sleep. Teens who fall asleep after midnight may still have to get up early for school, meaning that they might squeeze in only 6 or 7 hours of sleep a night. A few hours of missed sleep a night may not seem like a big deal, but it can create a noticeable sleep deficit over time.