Why do I feel tired?

If I go to sleep late around 3/4 am then I can wake up in the morning and become wakened after a walk outside, but then I could go back to sleep until about 1/2 pm. But if I go to sleep at a normal time 11/12 pm it's hard to get up in the morning, a walk won't always wake me up and I could sleep again until 2/3 pm and STILL feel tired... Why is this? How can I be able to wake up early and not feel tired all day and want to go back to sleep...?