1. An increasing desire or urge to put everything aside (at work, while reading, or whatever) in favor of a quick nap.
2. An instinctive physical desire, especially one for food and then a nap.
3. A sudden, uncontrollable desire to take a nap, which often occurs just after having a snack.
4. A term for someone who dreams about food while taking a nap.
5. Etymology: nap-, from Old English hnappian, "to doze, to sleep lightly" + [ap-] petite, from Latin appetitus, "appetite"; literally, "to strive for" or "desire for"; from appetitus, past participle of appetere, "to long for, to desire"; from ad-, "to" + petere, "to go to, to seek out, to aim for".

Together they usually mean, "a strong desire for a short period of sleep sometime during the day" in addition to a regular night's sleep. This, of course, depends on whether a person must work a night shift or has a regular day-time schedule.