It's frustrating to be tired in the middle of the day. Actually, it's frustrating to be tired at any part of the day other than when you're in bed trying to sleep.
On the other hand, is very fulfilling to be tired at the end of a day. Being tired when trying to sleep is like getting to the front of the buffet table after fasting all day.
The worst part of sleeping is when the alarm goes off. But if I wake up accidentally and see from the clock that it's not yet time to get up, I feel happy.