I have a question about the sleep cycle and the REM phase.

The WBTB methode assures that you fall into this phase right away.
Some dreamers stay up for an hour and the method still works, they pick up their cycle and dream on.

What if you sleep only four hours in the night, stay up for, let's say, eight hours, and go to bed early? Are you going to be REM-ing right away because your cycle isn't finished yet? (Probably not; you'd be awake for way to long but I'm just trying to explain what my question is)

So I am wondering: for how long can you stay up before the sleep cycle "resets" itselve and you start from the beginning again, and is this being influenced by the amount of sleep you had?