There is growing evidence that ADHD and Narcolepsy are basically the same condition, expressed in different ways. Or, more to the point, that ADHD is fundamentally a sleeping disorder just as Narcolepsy is. It’s not a widely accepted idea at this point, and it may not pan out to be true. The fact that the same medications work on both is a good reason to at least continue researching this hypothesis.
Worth an internet search for anyone interested.