It's not exactly 90 minutes in everyone, all the time – this is just an approximate average. It's always going to be harder to wake up when you're in the deepest stage of sleep, but when this occurs will vary. If your current sleep pattern works for you, I'd try to stick to it as far as possible.
Is it better to get sleep in multiples of 90 min, or to get as much sleep as possible?
Barry D Cooper 