Each sleep cycle takes 90-110 minutes. Each cycle has 5 steps. So each step takes about 20 minutes. But if your cycle takes 90 minutes your first step will take

Each sleep cycle takes 90-110 minutes. Each cycle has 5 steps. So each step takes about 20 minutes. But if your cycle takes 90 minutes your first step will take about 18 minutes. So if you wake up after 20 minutes you will be in the first 2 minutes of the next step. And that's a bad moment to wake up. As people already noticed, you have to play a little bit with the timing.