Real and Barca are set to clash once again as both teams booked their place in the quarterfinals of the Copa del Rey this week.
The two legged tie will mark the third and fourth time - they have previously played each other twice in the Super Copa and once in La Liga - the two bitter rivals have met this season with still half the campaig to go.
This follows on from some particularly unsavoury encounters between the sides in last season's UEFA Champions League semi-final as well.
“When there are few it's agreeable [but] when there are a lot you get tired [of them],” Casillas told Mundo Deportivo.
“In my opinion, as we’ve played so many games, sometimes feels a little decaffeinated.”
Meanwhile, Barca defender Gerard Pique expects his side's upcoming Cup clashes with Madrid to be close affairs.
"In a match between Barca and Madrid there are no favorites, it does not matter how they have been faring, we will go to the Santiago Bernabeu to try to win and get the best result possible," Pique told AS.
"This is what the draw decided, we are going to have to prepare very well, as we always do, and try to reach the semifinals and win the tournament."
Barca will play the first leg of the tie at the Bernabeu on January 18, with the return leg taking place one week later.