The real flaw in that theory is that ninety minutes is some kind of an average. My understanding is that there's a fair amount of variation between different people. There may also be variation between different sleep cycles of the same person. Unless you know a lot more about your own sleep cycles, I'm not sure how useful it is to plan around their expected length.

(I've played with trying things like that too. I'm not sure how successful it's been in actually having me wake alert more consistently.)