A death nap is a nap where you actually are probably in a coma. A nap so glorious that you accidentally doze off at 3 pm watching reruns of TLC's "What Not to Wear," and when you wake up it's pitch black in the room and you realize it's 4 am and you slept the entire day/night away. Then you can't fall back to sleep, so you watch home improvement shows or infomercials until you have to get ready for the day. Then you're so tired when you get home you repeat the cycle all over again.
"I was supposed to study for my final last night but I took a death nap and only had 3 hours left before the test."