Normally, we advise people to take an afternoon break when visiting the Disney World amusement parks. Go back to your resort room, take a nap, watch some TV, catch up on reading, whatever gives you some relaxation time. Disney parks can be amazingly taxing on the body and going all out, all day can really ruin that day and the whole vacation.
However, during the busiest times of the year (like right now -- the time from Christmas to just after New Years), Magic Kingdom and sometimes other parks will close due to capacity. Sometimes the closings happen pretty early in the day, sometimes in the afternoon. So, if you were to take your normal afternoon break, as we recommend, you may get "locked out" of the parks due to capacity restrictions! What a way to really ruin your vacation!
In this case, it's still a good idea to take an afternoon break, just do it in the park. Find a cool spot (on December 29th of this year, it was nearly 80 outside) and a chair and kick your feet up. Watch the crowds. Stay a while. 
This way, you can still go back to enjoying the park, not get locked out, and get some rest in the meantime.
We hope you had a very happy holiday season and New Year's!