I want to sleep only 6hours per day. How can I do it and still feel good during the day?

Sleep. Don't focus on the hours of sleep. Focus on your work & life chedule, proper eating, exercise routine. The reality is if you need 8 hours, then 8 hr it is. While awake, maxime you daily tasks. Your doctor may need to evaluate you several time before figuring out what is best for you depending on your health, stress level, and personal issues. Or goals.
TYPICALLY, WHEN WE. Try to fight and alter nature, we end up losing. The best advice I have for you is to be very consistent on a daily basis with the time you wake up every day. Bedtime should be when you feel tired and sleepy. I am sure with time, you could get very close to achieving your goal, but do not get frustrated with it. It will take a bit of time, sometimes months to set yourself up in a specific timeline.