ADHD

"That's your battle reflexes. In a real fight, they keep you alive. As for the attention problems, that's because you see too much, Percy, not too little."

- Annabeth Chase

ADHD stands for Attention-Deficit Hyperactivity Disorder. This is characterized by high levels of inattention, being highly distractible, impulsive and restless, hyperactive or hypersensitive. Most demigods are labeled as suffering from ADHD, but it really is a sign of their heightened senses and aptitude for battle. It is usually coupled with dyslexia in demigods.

Percy Jackson and the Olympians
In the series, ADHD is present everywhere from The Lightning Thief to The Last Olympian. Grover tells Percy that ADHD makes him inattentive to other things whenever he is in class. Grover also says that the instinct makes him want to get up and get outside to release that energy.

The Heroes of Olympus
All introduced demigod characters, with the exception on Frank Zhang, have ADHD. Leo Valdez, on an unusual note, has ADHD but no mention is made if he has dyslexia.

Trivia

 * ADHD makes demigods more aware of their surroundings, as stated by Annabeth.
 * The idea of demigods having ADHD and dyslexia came from Rick Riordan's son, Haley Riordan, having both problems.