Narcolepsy is a sleeping disorder in which a person tends to fall suddenly due to the unusual brain chemicals and signal.This can cause excessive sleeping, drowsiness at daytime, hallucination,loss in muscle tone, sleep paralysis etc.This disorder might occur due to genetic reasons.
Accordionist to the question's situation, Nolan might be suffering from narcolepsy as he felt asleep in the middle-of-the-conversation of vacation planning .This can be due to extreme and sudden sleep requirement which cause random eye movement sleep.
