He woke up screaming the first couple of times when the nightmares started. His brother right there, reaching out at him to strangle him, why did you kill me? written on his face, and Arganan could do nothing but scream and scream and scream because he didnโt know what to do, and then he would wake up.
This would forever haunt him. Killing his own brother (Calistaโs father) would forever haunt him. And if there was any way for the nightmares to stop, he would take it despite its risks. Trying to direct his focus somewhere else didnโt help too well, but over time he taught himself not to scream so much in his sleep, so that he didnโt scare everyone else in the castle.
But even then, it still hurt so much.