Once we got to the MK when it opened and stayed all day. We did the Family Magic Tour, had dinner at LTT. This was on Christmas Eve. We did the afternoon parade. We did it all. It was our last full day at WDW and we were leaving the next day. We usually leave around lunch time and drive part way home and finish up the second day.
Well, we went back to the hotel about 8 or 9PM. We got our son to bed, put out the Christmas presents that we brought along and went to bed. We opened presents, ate breakfast and then decided to just drive home. We were totally worn out by the long day the day before.
That said, we never spend the entire day at a park. We spend a few hours, go for a swim or rest before heading back to the theme parks. It's more relaxing and refreshing.