You don't have to try to wake up, you already do. What you're trying to do is notice yourself waking up. You often wake up after your dreams.

Keep in mind that sleep cycles aren't always 90 minutes. From my experience, they range from as high 2 hours to as low as 15 minutes. For me, they usually range from 1 hour to 30 minutes later in the night.

If you just pay attention to what happens while you're sleeping, you'll notice the stuff that you should.