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1.
The indescribable (post-nap) feeling like one has just completed physical exercise, even though they were sleeping the whole time. It is unclear how or why they sweat when they sleep, just that they wake up slightly dirtier than before they went to bed. Usually accompanied by a foul odor similar to urine.

Characterized by heightened heart-rate, fear, the need to shower, and possible alienation from their friends.
"Damn bro, Jim looks like she just went to the gym but she's in her pajamas. I think she's got Nap Sweat"

"Did Alvin just pull a nightfap or does he have some mean Nap Sweat going on?"
by RatchetRach April 12, 2013