Hey I'm a lot older than you and I have to get at least 7 full hours of sleep a night. I can remember when I was your age I always needed a lot more. Don't worry, it's normal for teenagers to need lots of sleep to make up for the difficult job of compensating for growth spurts.
Well lets see!! I cant sleep with just 6 hours nightly, I have to sleep for at least 8-10 hours even though I'm older than you. So since your getting 6 hours of sleep your missing some hours worth of sleep, it is normal since your a growing Young man, when you get older you'll decrease on the amount of sleep, but right now sleep as much as 8-10 hours and see if that works. If this does not work you will need to go to the doctor's because you may have a medical condition that needs to be looked at. (ex.Mono)
What's going on is that a 15-year-old needs AT LEAST 8-10 hours sleep, & possibly more, especially when they have been exerting themselves or stressing themselves with, say, lots of studying. So, for goodness sake, get enough sleep!
are you getting restful sleep are sleeping with a tv on or your favorite music playing these may help you to relax but once asleep these noises can keep the mind active not allowing for restful sleep and you may also need to look at your diet what you eat can affect your energy levels
It seems your needs are normal, but if you regularly need more than 12 hours of sleep and still feel sleepy there is a name for it. My daughter has it, but I can't remember the name. Only way to be sure is to have a sleep study done.
The National Sleep Foundation states that teens need on average 8.5 - 9.25 hours of sleep per night. That is average! You may need more depending on your other activities. If you sleep only 6 you deprive yourself of a whole night of sleep every four days! No wonder you are tired.