He's been spacey ever since the accident that was responsible for his "sleigh name" Comet. It was at the end of the run on the very first Christmas sleigh delivery. At that time, Comet was leading the team through a dark and foggy night.

He suddenly thought he spied the big Lighthouse light on the tip of the North Pole, the one they still use to guide the sleigh team home. Unfortunately that was in the days before radar and meteorological forecasts.

Yep, it was a real comet speeding through the edge of Earth's atmosphere, and a disaster was narrowly avoided after Comet head butted it out of orbit and saved the team. He became a hero, but he's still seeing stars, even in the daylight!