When you take a nap and don't get out of bed. Usually do to lack of sleep the previous night (s).
Due to the fact that you will wake up several times over the 5-12 hours of napping in, you do not get very much rem sleep. This is bad and is one reason why we do not nap in every day.
Josh hasn't slept in, in weeks. He gets about 5 hours of sleep a day. What does he decide to do at 5 p.m.? He decides to take a nap. After a few hours he wakes up and says, "I don't feel like getting up and facing the world, just a few more minutes." He falls back asleep and the cycle repeats. This should be a wake up call to Josh, he will probably get more sleep at night so he doesn't nap in again.