Getting 8 hours of sleep per night is good for most people but since you're still feeling tired even after 8 hours of sleep it could be that you need more than 8 hours of sleep per night.
Some people need up to 10 hours of sleep to feel energized the next day.
You can help yourself sleep better and longer by keeping electronics away from you when you go to bed, avoid caffeine a few hours before bed, keeping your bedroom quiet and dark and even using some white noise.
It could also be that you have a sleep disorder and even though you're in bed with your eyes closed and you feel like you're asleep maybe sometimes during the night your body is actually awake even though you've been in bed for 8 hours a night.