Light Yagami/Harry Potter
Light's mind tended to latch onto anything to stave off boredom. As a baby, there was only so much his mind could do for mental stimulation. Going over maths, for once. Took him two months and a week. Trying to recall various books he had read over his lifetime- Gods, Kira, he was only 23 when he died- only took him six months. Going back over the thirteen languages he had learnt before his untimely demise? Hardly a month.
Which meant it was only a year since his death, and subsequent taking over of a random British infant's body, that Light's mind started doing introspection.
Light had never had a nightmare before. Even after he killed people during the day with his death note, even after his father's death and Sayu's trauma, even after all that Light slept peacefully.
But introspection ruined it all.