I find that even if I do the 20-minute power nap, where you're not supposed to fall asleep, not supposed to get groggy, I find myself slipping into a deeper sleep and wanting to sleep for two or three hours.
So instead of your body prepping to wake up, it's going in the opposite direction and as a result the second alarm may cause you to feel even more tired and so continues the vicious cycle.
You know, after work all you want to do is throw yourself onto your bed. And during the weekend, you just want to stay at home and enjoy a family dinner.