I have tried to wake myself up by, before going to sleep, telling myself repeatedly that I will wake up at a certain time while visualizing that particular time in a variety of formats (four in the morning, 4am, 4 o'clock in the morning, etc) and I did it subliminal message style (like, flashing it in front of my eyes randomly) and I've woken up in the middle of the night every single time. Curious thing, the brain is.