Basically thus is called shifting of sleep cycle, usually people tend to set the time to go to bed rather than time to get up which is the main reason. Brain memories sleep clock and sets the same time of getting up the next day also. This is a simple problem given no other medical illnesses to rectify.
Next Steps
maintain good sleep hygeine - you can google for tips, if you cant find please consult
do not take nap or sleep in the daytime (afternoon)
set the getting up time by 6am irrespective of the time you sleep,