When i sleep 7-8h i still feel pretty tired when waking up. with no alarm, i can sleep 12-14h. i don't snore or wake up frequently during the night. what could be wrong?

See a doctor: Sleep is very important and getting into all phases of sleep as well. You should talk to you doctor and see if need to have a sleep study done.

GOOD for you!: It is a bit long hours of sleep requirment for your age. Usually 8 to 9 hours are sufficient. If you are involved in intense Physical Exercise or having lots of stress, some times Sleep becomes the escape route. if you are healthy and not have any physical or Mental stress, & not on any medicines (Side effect) May check for Anemia Vit . Deficiency and Thyroid Function tests with your doctor

Depression?: While much has been written about sleep deprivation/getting too little sleep, much less is known about the health impact of too much sleep, typically defined as >9 hours/night. Usually this is associated with some confounding medical condition: depression, sleep apnea, substance abuse.If none of these are present, there seems to be little adverse health impact. Can have depression without sadness!

Get a sleep study: Sleep studies are tests that watch what happens to your body during sleep and how your body responds to sleep problems. See your PCP for consultation and sleep breathing disorder evaluation