I had this problem recently when I had to pass exams at university.
From what I know, the ideas we think about before going to bed is the ones that our mind considers the most important among the ones we generated throughout the day,
So, we need to think about waking up early before going to bed.
Plus, ideas expressed with the words written on a piece of paper are 10 times much stronger than the ones spoken or thought about.
So, I just wrote on a piece of paper that I needed to wake up at 5 o'clock before going to bed.
And it worked for me perfectly each time.