ANDY'S ANSWER: Teenagers need a minimum of 8 hours of sleep per night in order to to function at their best. If your teen has to get up on school days at 7 am, I would say that lights out should be no later than 10:30 pm, to give them time to wind down and get to sleep. It's probably even best to be in bed at 10 pm and read or listen to music for a half hour. My parents always gave me a bedtime that was the time I had to have my teeth brushed, pajamas on, and actually be in bed. The lights got to stay on for an extra half hour past that for reading right up to my high school years. When I was getting older, as my bedtime was extended , my half hour reading period was also extended. On weekends or holidays, I was allowed to stay up an extra hour, and I was allowed to sleep in an extra hour.
Keeping to a schedule with sleep routines is also far better for your teen than allowing them to stay up as long as they want on weekends. Better to perhaps allow them to stay up a maximum of 2 hours more than their regular bedtime and allow them to get up a maximum of 2 hours later in the morning. Letting them stay up as late as they want will result in them sleeping in for a large part of the day. If they've slept until noon or later on Sunday morning, it's going to be pretty hard to get them in bed at their proper bedtime on Sunday night. That will mean that come Monday morning when they need to be up early again, they will certainly be far from their best from not getting their proper sleep the night before. Sure, there will be special occasions when staying up later will be necessary or allowed, but as a matter of course, you should be trying to maintain a somewhat stable schedule for your teen.
There are certainly a lot of things that will keep your teen from sleeping, even if you do send them to bed on time. TV's, computers and cellphones in the bedroom are the replacements for reading under the covers with a flashlight from past generations. Teen will text each other well into the wee hours of the morning if you allow them access to their electronic devices after bedtime. The challenges of parenting in a wired world are certainly nothing less than daunting, but the consequences of your teen not getting enough sleep each night can affect everything in their lives.
Sleep deprivation affects physical health, lowers the body's natural immunities and can contribute to weight gain. Mental health is also affected, resulting in higher instances of anxiety and depression. Those who are not getting enough sleep will be irritable, quick to lose their tempers and make poorer choices in their decision making. Concentration and the ability to retain information is also impaired when the body has not had sufficient rest. Although this is true for everyone, it is especially critical for teens whose minds and bodies are still growing and developing.