Many teens have a difficult time falling asleep because their natural sleep cycles fight against their schedules. Teens need around nine hours of sleep a night and their natural sleep cycle pushes their fall-asleep hour to fairly late. Unfortunately, teens' busy schedules and early school starting times conspire to force teens into a much earlier bedtime, when they're not naturally sleepy.
First, make sure your teen goes to bed and wakes up at the same time each day, including weekends and vacations. This can be quite difficult to do, but switching between different sleep cycles makes it difficult for teens to acclimate to either of them. You can also encourage your teen to make their bed a sleep-only zone. They can read, use their laptop or talk on their cell phone elsewhere in their rooms. If they keep their beds for sleeping only, their bodies will start getting into sleep mode once they get into bed. Even better, teens should stop using electronics about an hour before going to bed. Taking this last hour to wind down and to slow their brains down from all these stimuli helps transition teens into sleep mode.
Teens are drinking more caffeine than ever, so try to limit their intake. In particular, teens shouldn't be drinking caffeine within two hours of going to bed. Also consider how overbooked your teen is with activities and schoolwork. If there truly aren't enough hours in the day for your teen to get through it, work with your teen to prioritize. Some activities may need to be reduced or eliminated.
If your teen has an underlying medical condition and is taking medication, talk to his/her physician to see if sleep difficulties can be related either to the medical condition or the medication being taken. It's not recommended that teens take medication specifically to help them sleep, although some medical conditions, such as depression or behavioral problems, are treated with medicines that can help teens sleep. Finally, consider if your teen's sleep issues are just symptoms of a larger problem, such as trying to avoid school, depression, or drug use.