If Matilda's trauma is an indication, Clark's nightmares must last a long time.

Poor winners. They can never be at peace with their consciences.
Andreia
In real life, PTSD can last for a lifetime. A casual observer might wonder why a person doesn't just get over it and move on, but it isn't that simple. Those painful memories can be close to the surface, or more deeply buried, but sometimes, something will trigger those memories, and they come back to bite.