Are you sure you're not still waking up in the middle of the cycle? 90 minutes is just a loose generalization. Different people have different sleep cycles and

Are you sure you're not still waking up in the middle of the cycle? 90 minutes is just a loose generalization. Different people have different sleep cycles and take different amounts of time to fall asleep. Personally, 2 hours works well for me but 90 minutes does not.