In in the 1970's researchers at Stanford University discovered that teenagers require more sleep, by 1 to 2 hours, than do their younger 9 and 10 year old siblings, yet most teenagers get 1 to 2 hours less. Teenagers are the sleepiest members of society and this sleepiness is associated with poor school performance, increased drug and alcohol use, and increased automobile accidents. This level of sleepiness may also play a role in the high rate of teenage suicides. However, in addition to requiring more sleep than do 9 and 10 year old children, or adults, teenagers typically have altered biologic rhythms which vary the time of night teenagers sleep best and the times of day teenagers are most alert. The timing of sleep and wake is very dependent on the sun.
Humans typically sleep at night and are awake in the daytime.
If teenagers sleep well at night, they are very likely to function well in the daytime. The Sleep Center at St Mary's Hospital in Walla Walla, Washington has an excellent discussion on this subject. You may find the results of our on going poll useful in letting your children know what other parents think about bedtime.