It is not normal. The normal sleep for a teen is 8 hours. This is something that a doctor needs to take a look into. Make an appointment and drag her down if you have to. There is something wrong here.
As I am a mother of seven and grandmother of sixteen, I have a few ideas to share. My first question is: Has anything stressful occurred recently--at school, at home, or in her relationships with peers? Next, in adolescents, depressive symptoms are often much different than those which you would see in adults. Don't ignore anything. None of my kids, as teens, overslept in this manner. I would not consider it normal, until I had consulted a medical professional. Most girls avoid confiding in their parents at this age, unless real patterns of communication have existed from childhood. Even then, peers usually outstrip parents in most areas of importance. So, don't panic--just calmly seek out some advice and don't make the mistake of revealing all of your intents and purposes to her at this time. Best wishes for you and your daughter.
I know my teenagers sleep at most 12 hours a night while they are growing. When my oldest reached 18, she finally started sleeping about 8 hours a night. It sounds like your daughter needs to be checked out by a doctor. My friend's daughter was 14 years old when she was sleeping too much. She's back to normal now, but her periods had stopped coming for about 6 months - this seemed to be triggered by her grandmother's death (the grandmom lived in the house with them) and she slept about the same amount of time as your daughter. Her daughter stopped interacting with her friends and just wanted to sleep, sleep, sleep. Then her period started coming again and within about two months or so she returned to her normal self.
There could be many reasons for the excessive sleepiness. A doctor will hopefully be able to figure out the cause of it. Hang in there!
I have twin boys who are 14, I have noticed that they are sleeping a lot more than they used to. They aren't sleeping 16 hours though, probably about 11-12 hours, and sometimes they will nap for an hour in the afternoon, my mum says it's their age and hormones.
They are very active children though, always have been, they love sports and run and bike everywhere..they have reached an important part of their education, they have done GCSE exams this year 2 years early..I think that mixed with hormones, sport and school it has just worn them out and they need extra sleep to cope with it all, I am one of those people that needs a lot of sleep so they must be like me..
I was worried about it though, I was thinking allsorts, that they'd been drinking or taking drugs!
If you can't see any reason for her excessive sleep then it points to a problem, although even if she does have a lot going on 16 hours is still a lot.
Straight away when I read your post I thought of Anaemia.. does she have heavy periods? When I was in my teens I used to have fainting spells, and after loads of tests it was put down to my hormones and periods, and I was only borderline Anaemic.
I would take her to the doctor and get some blood tests for Anaemia, Thyroid, Diabetes etc..
There are lots of things that can cause excessive sleep, hopefully it's not depression, but it can be treated, I would hate to be a teenager again, it's such a difficult time..some people have a harder time than others though.
Good Luck, keep us posted!
I also dont think it is normal for her to be that tired , does she seem anxious or depressed about anything, what does she do during the day ? a visit to the doctor may be a good idea, also try to motivate her by giving her some tasks that may help you around the house, has she any friends?