[Divini]

Spoiler for Comparison to the other Key anime:

If there's anything I learned from watching the other Key-adapted animes (Air, Clannad, Kanon), is that there's always a supernatural element that plays a significant role in the story.

Riki's narcolepsy just screams that plot element. All I can guess is that it's related to whoever wrote that note tied to the cat, and/or related to what the "secret of the world" written on that note is.