I still feel groggy after a 90 minute sleep; it's not as bad as waking up in the middle of the cycle, but I still feel groggy. If I nap for 3-15 minutes, I wake up feeling as awake as I am during exercise.
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.
