you have take some form of drugs and on the day after you're on a comedown - you're in public, you're paranoid, you're sleepy. your mates look out for you so you can have a sketchy (aka sketching out as an after effect of drugs) nap time - so to avoid eye contact and generally to catch up on sleep deprivation - you have a sketch nap.
In a cafe being very quiet and sketching out - feeling very tired. your mates suggest a public nap - because you're not comfortable enough to fully fall asleep but your body needs rest - "mate, have a sketch nap"
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