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When one uncontrollably falls asleep every time they lay down flat, parallel to the floor, even if it is not a planned "nap time."
Roger: "Hey man, why weren't you in class today?"

Matt: "Dude, i laid down on the couch to watch TV and my horizontal narcolepsy kicked in... woke up 4 hours later."
by midleman08 May 11, 2010
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